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From Midnight–Noon Sleep to Biphasic Sleep: Night-Shift Workers Don’t Just Need More Sleep—they Need a Measurable Biological Time Structure

July 24, 2026

Insights from Roger Sexton’s BBC Feature: CIE, WELL, and Eye-Level Mel-EDI Measurements

At 4 a.m., the hospital ward begins to quiet down. A resident physician has been working for nine consecutive hours. With physical fatigue setting in and her eyes strained, she looks forward to returning home at 6 a.m. and recovering with sleep.

However, once she enters a darkened bedroom and closes the blackout curtains, her biology delivers the opposite message: morning has arrived, and the body is preparing for wakefulness.

This paradox is at the heart of the BBC article “The hidden cost of the night shift and how to sleep it off,” shared recently by Roger Sexton, Board Member of the Good Light Group (GLG). The challenge faced by night-shift workers is not merely insufficient sleep; it is the disruption of an intrinsic circadian timing system shaped by human evolution.

The discussion also raises important questions for lighting science and healthy building design: how can standards such as the CIE recommendations and WELL Building Standard, together with measured eye-level mel-EDI (melanopic Equivalent Daylight Illuminance), help create environments that better support people working against their natural biological clock?

Source: BBC InDepth, Pallab Ghosh
Original article: https://www.bbc.com/news/articles/cp9errxl97go

Roger also shared his own experience. After retirement, no longer needing to wake up at 7 a.m. every morning, his body gradually shifted into a form of biphasic sleep: sleeping for one period at night, naturally waking around 1 a.m., making a pot of tea, and then returning for a second period of sleep.

This is not a clinical study, but it raises an intriguing question: have we become too accustomed to viewing “sleeping straight through until morning” as the only correct way to sleep?

Rather than simply extending daytime sleep duration, the goal may be to redesign the interaction between sleep patterns, light exposure, and the body’s internal biological clock.

The Hidden Cost of Night Shifts Is Not Just Sleep Deprivation

Sleep is regulated by two interacting systems. The first is sleep pressure: the longer we remain awake, the stronger the drive to sleep becomes. The second is the circadian rhythm: the body’s internal biological clock that promotes wakefulness at certain times and sleepiness at others.

At the end of a night shift, workers may have accumulated substantial sleep pressure. Yet at the same time, exposure to morning daylight, the commute home, and the body’s own awakening signals are all sending the brain the same message: “A new day has begun.”

As a result, some night-shift workers, despite feeling exhausted, find that they can sleep only three or four hours after returning home before waking naturally. Blackout curtains can reduce light exposure in the bedroom, but they cannot completely erase the morning light that has already entered the eyes during the journey home, nor can they immediately reverse the body’s internal clock.

Roger also highlighted the BBC article’s discussion of cortisol and insulin resistance. More precisely, sleep restriction and circadian misalignment may work together to alter cortisol rhythms, sympathetic nervous system activity, inflammatory responses, and glucose regulation. Cortisol is one possible pathway involved, rather than a single explanation for all health risks associated with night-shift work.

The true disruption caused by night work is therefore not simply a loss of sleep, but the breakdown of an entire biological order:

  • The body needs to sleep, but the job requires wakefulness.
  • The body prepares for wakefulness, but the worker is only beginning to sleep.
  • Food is consumed during the biological night, while rest occurs during the biological day.
  • Workplace lighting is used to maintain alertness, but the light encountered after the shift may continue delaying sleep.

Sleep quantity matters, but circadian alignment depends equally on sleep timing, pre-sleep light exposure, and post-awakening light signals received by the eye.

What Can Ziwujue Teach Modern Night-Shift Workers?

One of the earliest discussions of sleep timing appears in The Yellow Emperor’s Inner Canon (Huangdi Neijing, Lingshu, “Discussion on Great Confusion”), where sleep and wakefulness are explained through the alternation of yin and yang between day and night:

“Wei Qi circulates through the yang during the day and through the yin at night. Therefore, when yang is exhausted, one falls asleep; when yin is exhausted, one awakens.”

Original source: Huangdi Neijing · Lingshu · Dahuo Lun
Original text: https://ctext.org/huangdi-neijing/da-huo-lun/zh

Although expressed in the language of traditional Chinese medicine, the passage conveys a simple but enduring understanding of biological time: the human body changes with the cycle of day and night, and the restorative value of rest cannot be separated from when it occurs.

Several centuries later, during the Qing Dynasty, Cao Tingdong described this idea more explicitly in Lao Lao Heng Yan (Essential Sayings for the Elderly):

“Sleep during the daytime… and sleep again at night; within one day and one night, divide sleep and wakefulness into two periods.”

Original source: Cao Tingdong, Lao Lao Heng Yan (Qing Dynasty)
Original text: https://ctext.org/wiki.pl?chapter=75486&if=gb&remap=gb

Cao also discussed taking a brief rest around midday while maintaining deep sleep during the Zi period (around midnight). However, these recommendations were written primarily for older adults rather than as a general prescription for today’s night-shift workers.

For this reason, we should avoid reducing Ziwujue to simplified claims such as:

  • Missing sleep during the Zi hours prevents the body from “detoxifying.”
  • Skipping a midday nap prevents the heart from repairing itself.
  • Yin is equivalent to melatonin, while yang is equivalent to cortisol.
  • Modern biphasic sleep has “proven” the ancient theory of Ziwujue.

Such claims are supported neither by the classical texts nor by contemporary circadian science. They also force an overly simplistic correspondence between two fundamentally different systems of knowledge.

Nevertheless, Ziwujue still offers a framework worth learning from. It suggests that a 24-hour day may include one principal sleep episode together with a second opportunity for recovery. In other words, healthy sleep depends not only on how long we sleep, but also on when we sleep.

We might therefore reinterpret Ziwujue as a functional Ziwu framework:

  • The biological “Zi” represents the primary or anchor sleep that should be consistently protected.
  • The functional “Wu” represents a second recovery opportunity—whether a scheduled second sleep period, a pre-shift sleep session, or a strategic nap adapted to one’s work schedule.

In this framework, Zi and Wu need not be rigidly tied to 11:00 p.m.–1:00 a.m. and 11:00 a.m.–1:00 p.m. For night-shift workers, what matters more is translating clock time into biological time.

Ancient physicians described this principle through the waxing and waning of yin and yang. Modern science explains it through the interaction between sleep pressure and the circadian rhythm. These are not the same mechanisms, yet they converge on a common insight:

The human body is not a work schedule that can simply be rewritten.

Can Biphasic Sleep Be Part of the Solution?

The BBC article highlights research by Norwegian scientist Line Victoria Moen, who studied night-shift workers in the Arctic. She found that some workers did not sleep continuously for seven or eight hours after their shift. Instead, they slept from around 9:00 a.m. to 1:00 p.m., followed by another sleep period before returning to work. This is also a form of biphasic sleep.

However, the term biphasic sleep currently encompasses at least three distinct patterns:

  • The historical pattern of a “first sleep – period of wakefulness – second sleep.”
  • The night-shift pattern, consisting of a main daytime sleep followed by a pre-shift recovery sleep.
  • A planned on-shift nap, typically lasting 20–50 minutes.

Although all three are described as biphasic sleep, they differ in physiological mechanisms, purpose, and duration.

Moen and her colleagues are still systematically reviewing the available evidence, and there is currently no universally accepted definition of biphasic sleep in the scientific literature. At present, the evidence supporting planned naps for reducing immediate sleepiness and improving alertness and job performance is considerably stronger than the evidence that biphasic sleep reduces long-term risks such as cardiovascular disease, cancer, or dementia.

Research references

Biphasic sleep may therefore be a valuable recovery strategy, but it should not be promoted as a universal remedy capable of eliminating all of the health risks associated with night-shift work.

The CIE Perspective: It’s Not Just About Light—It’s About the Right Light at the Right Time

The International Commission on Illumination (CIE) reminds us that light is more than a means of seeing our surroundings—it is also one of the body’s most powerful biological time cues.

In its updated 2024 Position Statement, the CIE summarizes this principle in a simple phrase:

Proper Light at the Proper Time

Position Statement
CIE PS 001:2024, Recommending Proper Light at the Proper Time (3rd Edition)
https://files.cie.co.at/CIE%20PS%20001_2024%20CIE%20Position%20Statement%20%20Recommending%20Proper%20Light%20at%20the%20Proper%20Time-3rd%20ed.pdf

Using the metrology defined in CIE S 026, we can quantify the circadian-effective light reaching the eye using melanopic Equivalent Daylight Illuminance (melanopic EDI, or mel-EDI), expressed in lux (lx). Rather than measuring the amount of visible light alone, mel-EDI estimates the light stimulus reaching the eye that is relevant to the melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs).

Standard
CIE S 026:2018 – CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light
https://www.cie.co.at/publications/cie-system-metrology-optical-radiation-iprgc-influenced-responses-light-0

For healthy adults aged 18–55 with regular daytime schedules, the expert consensus adopted by the CIE recommends:

  • At least 250 lx mel-EDI at the eye during the daytime.
  • No more than 10 lx mel-EDI during the three hours before habitual bedtime.
  • No more than 1 lx mel-EDI during sleep. If visual tasks are unavoidable at night, light exposure should be kept as low as practical while maintaining visual safety.

Expert Consensus
Brown et al., Recommendations for Daytime, Evening, and Nighttime Indoor Light Exposure
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3001571

These values are research-based reference targets, not medical safety thresholds. Nor are they prescriptions that can simply be transferred to night-shift workers.

It would be tempting to assume a straightforward translation:

  • ≥250 lx mel-EDI at the beginning of a night shift,
  • ≤10 lx before going home,
  • ≤1 lx while sleeping during the day.

But this is not the strategy proposed by the CIE.

CIE technical guidance explicitly states that these recommendations are intended primarily for day-active populations. For people who must remain active at night, lighting strategies should be individualized according to work schedules, non-work activities, and the person’s complete history of light exposure.

Technical Report
CIE TN 015:2023
https://files.cie.co.at/CIE_TN_015_2023.pdf

A permanent night shift, several consecutive night shifts, and rapidly rotating shifts all require different lighting strategies. Effective circadian lighting for shift workers must consider the entire 24-hour “light history”—including work, commuting, pre-sleep periods, daytime sleep, post-awakening exposure, and days off.

In other words, the CIE provides us with a measurable language for circadian light—but not a universal timetable for night-shift workers. Shift workers need both sufficient light to work safely and protected darkness to recover effectively.

The Value of WELL: Moving the Focus from Luminaires to the Human Eye

One of the most important contributions of the WELL Building Standard is its shift in perspective: evaluating lighting not by what luminaires emit, but by what building occupants actually receive.

The current WELL v2 Light Feature L03: Circadian Lighting Design is explicitly titled:

Meet Lighting for Day-Active People

That wording is especially significant when considering night-shift environments. It reminds us that lighting targets developed for daytime occupants should not automatically be treated as prescriptions for night-shift workers.

For electrically lit daytime workstations, L03 specifies multiple performance thresholds, including:

  • 150 EML, approximately equivalent to 136 lx mel-EDI
  • 275 EML, approximately equivalent to 250 lx mel-EDI

These values must be verified at eye level on the vertical plane, rather than by measuring horizontal desktop illuminance alone. The standard also requires that the specified exposure duration be achieved and confirmed through on-site performance verification.

Standard
WELL v2 – Light, Feature L03: Circadian Lighting Design
https://v2.wellcertified.com/en/wellv2/light/feature/3

Performance Verification
IWBI – What Is Performance Verification?
https://support.wellcertified.com/hc/en-us/articles/25695299643671-What-is-performance-verification

Performance Verification Guidebook
https://resources.wellcertified.com/tools/performance-verification-guidebook/

Three practical details are frequently overlooked.

First, WELL’s Equivalent Melanopic Lux (EML) is not numerically identical to the mel-EDI defined by CIE S 026. A value of 150 EML should not be interpreted as 150 lx mel-EDI.

Second, WELL’s standardized performance verification measurements simulate eye-level exposure under defined conditions. In real workplaces, however, night-shift workers continually change posture—standing, sitting, looking down, or shifting their viewing direction—resulting in highly variable light exposure.

Third, WELL performance testing verifies the lighting performance of a particular space under specified conditions. It does not demonstrate that an individual night-shift worker receives optimal circadian light throughout a 24-hour day, nor does it directly predict health outcomes.

In short, standards define what should be achieved; field measurements reveal what has actually been achieved.

Why Luminaire Specifications Alone Cannot Guarantee Eye-Level mel-EDI

A lighting design can specify target values, and a luminaire datasheet can provide spectral power distribution (SPD), correlated color temperature (CCT), and illuminance data. However, the amount of light that actually reaches a worker’s eyes is influenced by many additional factors, including:

  • Luminaire location, beam distribution, and dimming settings.
  • Reflections from walls, floors, equipment, and surrounding surfaces.
  • Daylight entering through windows, as well as shading devices and curtains.
  • Obstructions such as partitions, monitors, medical equipment, or machinery.
  • The occupant’s posture, eye height, and primary viewing direction.
  • Whether the worker is facing the light source, facing away from it, or spending long periods looking downward.

Even within the same nurses’ station, using the same lighting installation and with identical desktop illuminance, two people—one facing a window and the other facing away from it—may receive substantially different eye-level melanopic EDI (mel-EDI).

The distinction between mel-DER and mel-EDI is therefore essential.

  • mel-DER (melanopic Daylight Efficacy Ratio) answers the question:
    For the same photopic illuminance, how effective is this spectrum at stimulating the melanopsin pathway?
  • mel-EDI (melanopic Equivalent Daylight Illuminance) answers a different question:
    At this specific location, viewing direction, and point in time, how much melanopic stimulus actually reaches the observer’s eye?

In simplified terms:

Eye-level mel-EDI = Eye-level photopic illuminance × mel-DER

Consequently, specifying only a correlated color temperature, reviewing only a luminaire’s SPD, or measuring only horizontal desktop illuminance is insufficient to determine a person’s actual circadian light exposure.

Two light sources with the same 4000 K CCT may have different spectral distributions and therefore different mel-DER values. Likewise, even if two luminaires produce identical output, differences in installation geometry, room reflectance, and viewing direction can result in very different eye-level mel-EDI.

Specifications are not exposure. Simulations are not delivery. Design intent is not measured performance.

Why In. Licht Belongs in the “Last Meter”

Ultimately, successful circadian lighting is not achieved simply by writing 250 lx, 10 lx, and 1 lx into a lighting specification. It requires verifying what actually reaches the occupant’s eyes.

Using In. Licht Ultra (Works with WELL™) as an example, the instrument can be positioned at the actual eye height of a night-shift worker, with its sensor oriented toward the person’s primary viewing direction. It measures the spectral power distribution created by the combined effects of electric lighting, daylight, and room reflections while simultaneously reporting:

  • Photopic illuminance
  • mel-DER
  • Eye-level mel-EDI

Works with WELL™ Program
IWBI Announces the Works with WELL Program
https://resources.wellcertified.com/press-releases/iwbi-announces-the-works-with-well-program/

It is important to clarify that Works with WELL™ indicates that a product or solution can support specific WELL strategies or performance thresholds. It does not mean that a single measurement performed with the instrument constitutes WELL Certification or replaces formal third-party Performance Verification.

For a night-shift environment, measurement should not be limited to a single location or moment. Instead, it should encompass the critical phases of the worker’s daily cycle, including:

  • Pre-shift rest or recovery-sleep environments.
  • Workstations where alertness must be established at the beginning of the shift.
  • Primary task areas during the circadian low point.
  • Spaces occupied before leaving work when the body should begin preparing for rest.
  • Break rooms, changing areas, and transition spaces during the commute home.
  • The daytime sleep environment and any unavoidable nighttime pathways within the home.

For WELL project pre-assessments, measurements should follow the standard’s specified eye height, measurement orientation, and sampling methodology. For operational studies involving real night-shift workers, however, measurements should go further by reflecting actual eye height, primary viewing direction, and the timing of real work activities.

The measured results can then be compared with shift schedules, lighting control scenarios, and project-specific eye-level performance targets before optimizing:

  • Dimming levels
  • Spectral tuning
  • Local task lighting
  • Luminaire aiming
  • Shading strategies
  • Time-based lighting control sequences

After adjustments are made, the space should be measured again.

This creates a continuous workflow:

Performance Targets → Lighting Design → Field Measurement → Control Optimization → Re-Verification → Operational Review

The value of In. Licht lies in supporting design verification, on-site field measurements, and ongoing operational review, transforming eye-level lighting requirements from design intent into measurable, documented, and continuously optimizable performance data.

A single measurement cannot represent an individual’s long-term light dose, nor can it predict health outcomes. However, if eye-level exposure is never measured, “healthy lighting thresholds” risk remaining little more than design aspirations.

Measurement does not replace science—but without measurement, science is unlikely to become operational practice.

A More Practical “Functional Ziwu” Strategy

For night-shift workers, restoring biological order requires more than simply increasing sleep duration. It calls for an integrated approach to sleep timing, light exposure, and circadian biology.

1. Anchor the Primary Sleep Period

Whether adopting a monophasic or biphasic sleep pattern, maintaining one stable, protected anchor sleep should be the priority.

The sleep environment should not only be dark, but also minimize noise, maintain a comfortable temperature, and reduce interruptions from family activities, phone calls, or electronic notifications.

For permanent night-shift workers, this anchor sleep should remain as consistent as possible. For workers on rapidly rotating schedules, however, it is generally neither practical nor advisable to force a complete reversal of the body’s circadian rhythm.

2. Make the Second Sleep Planned Rather Than Accidental

If the primary daytime sleep lasts only three or four hours, a scheduled pre-shift recovery sleep or strategic nap can help restore alertness before work. Adequate time should also be allowed for sleep inertia to dissipate before safety-critical tasks.

The objective is not to force a perfect uninterrupted sleep episode, but to maintain both total sleep opportunity and operational alertness more reliably.

This can be viewed as a modern interpretation of the traditional Wu sleep—a functional recovery sleep, although it does not necessarily occur at midday.

3. Night-Shift Lighting Should Not Remain Constant Throughout the Shift

During the early part of a shift and during safety-critical tasks, workers require sufficient visual illumination together with appropriate eye-level circadian-effective light exposure to support alertness and performance.

As the end of the shift approaches—and particularly when workers intend to sleep soon after returning home—unnecessary light stimulation should be gradually reduced to facilitate the transition toward sleep.

However, lighting adjustments must never compromise visual performance or safety. Medical procedures, driving, industrial operations, and public safety always take precedence over circadian optimization.

4. Don’t Turn “Avoiding Light” Into a Safety Risk

Morning daylight after a night shift may delay daytime sleep, but if a worker is severely fatigued, driving safely is more important than minimizing light exposure.

Circadian strategies should never encourage practices that impair visual performance while driving, such as wearing excessively dark lenses solely to “protect melatonin.”

Traffic safety must always come first.

5. Different Shift Patterns Require Different Strategies

Workers covering occasional night shifts or rapidly rotating schedules are generally not good candidates for attempting a complete circadian phase shift.

Those working permanent or consecutive night shifts, on the other hand, may benefit from more stable anchor sleep, carefully timed light exposure, and consistent scheduling on days off.

There is no single circadian lighting recipe that fits every type of shift work.

6. Continuously Improve Using Eye-Level Measurements

Record not only mel-EDI, but also:

  • Measurement time
  • Measurement location
  • Viewing direction
  • Lighting control settings

Where possible, combine these data with:

  • Subjective sleepiness ratings
  • Sleep duration
  • Work performance or error records
  • Wearable-device data
  • Other physiological biomarkers

Together, these data can create a feedback loop linking:

The lighting environment → Personal light exposure → Physiological and behavioral responses

This is the critical step in moving from healthy lighting concepts to verifiable healthy light environments.

From a Cup of Tea at 1 A.M. Back to the Human Eye

Roger’s personal experience does not prove that biphasic sleep is appropriate for everyone. What it does suggest is that the human body may not always be willing to conform to the uninterrupted sleep schedule imposed by modern industrial society.

The tradition of Ziwujue offers a cultural perspective that respects biological timing.

The BBC article brings greater public attention to the conflict between night-shift work and the circadian clock.

The CIE provides a scientific language for quantifying the biological effects of light.

The WELL Building Standard shifts the focus from lighting design intent to eye-level performance and field verification.

And In. Licht helps bridge the final gap between design criteria and actual human light exposure.

Individually, none of these elements solves the challenges of night-shift work.

What is truly needed is a 24-hour biological time framework that integrates work schedules, sleep planning, lighting control, eye-level measurement, and operational management.

Perhaps, in the future, we should stop asking:

“Is this a circadian lighting system?”

Instead, we should ask:

“At what time, in what location, and in which viewing direction, how much melanopic EDI actually reached this person’s eyes? And when recovery was needed, how much genuine darkness did we preserve?”

Night-shift workers have never lacked only an opportunity to catch up on sleep.

What they lack is a biological time framework that can be designed, measured, and verified.

Author
Lawrence Lin
Founder & CEO, Lighting Recipe Studio (LRS)
Chair, Good Light Group Asia (GLGA)
Board Member, Good Light Group (GLG)
IWBI WELL Light Concept Advisor

Disclaimer
This article discusses lighting, sleep, and built-environment strategies and should not be considered medical advice. Individuals experiencing persistent daytime insomnia, excessive sleepiness while working, fatigue-related driving risks, or symptoms suggestive of Shift Work Sleep Disorder should seek evaluation from a qualified sleep medicine or occupational medicine professional.

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