Guidance dated July 14, 2026, from the US National Institute for Occupational Safety and Health (NIOSH) says nonstandard schedules can disrupt or shorten sleep and calls for employers and workers to manage fatigue risk together. A May 2026 abstract of a board-approved guideline from the American Academy of Sleep Medicine (AASM), a US professional association, gives only conditional recommendations for shift work disorder and rates most light and scheduling evidence very low certainty.

The 2026 Yellow Book from the US Centers for Disease Control and Prevention (CDC) says correctly timed light exposure or light avoidance can help synchronize the internal biological clock. The direction matters: light can move sleep and wake times earlier or later, so more light at an arbitrary clock time is not a universal remedy.

Lane drift or microsleeps take priority over clock adjustment

NIOSH lists nodding off, slower reactions, lane drift, tunnel vision, microsleeps and forgetting part of the route just traveled among the effects of driver fatigue. A driver who notices any of these signs should leave the road safely and rest or change drivers; a worker operating hazardous equipment should stop and report the fatigue risk rather than rely on a light schedule.

NIOSH also advises screening for a health problem when fatigue persists despite adequate sleep. Repeated insomnia, unintended sleep episodes or sleepiness that interferes with work warrants clinical assessment instead of being assumed to be routine jet lag or an unavoidable part of shift work.

Light can move the clock in either direction

The CDC says bright light after the circadian low, which usually occurs about two to four hours before habitual waking, promotes a phase advance and can move sleep earlier; evening light generally promotes a phase delay and can move sleep later. “Morning” therefore refers to biological timing, not one fixed hour on a wall clock. A night worker and a daytime worker can be at different circadian phases at the same local time.

The US National Heart, Lung, and Blood Institute (NHLBI) calls light the strongest environmental signal for resetting the sleep-wake cycle and describes dimmer light before planned sleep as one part of treatment. Natural daylight, room lighting and a clinical light box are not interchangeable exposures, and the source does not establish one brightness or duration for every schedule.

A person stands outdoors in early morning sunlight (illustrative image)

Eastbound and westbound travel use opposite goals

The CDC defines jet lag disorder as sleep disturbance or excessive sleepiness with impaired daytime function after rapid travel across at least two time zones. Eastbound travel usually requires moving the body clock earlier, while westbound travel usually requires moving it later.

The Yellow Book says shifting sleep about one hour per day toward destination time for two or three days before travel may reduce the adjustment still required after arrival. That means earlier sleep and waking before an eastbound trip and later sleep and waking before a westbound trip, when the schedule makes preparation practical.

During travel, the CDC advises timing sleep, meals and light to the destination, avoiding excess alcohol, and using natural light at the destination while avoiding lengthy daytime naps. It also says optimal light timing depends on both direction and the number of time zones crossed, so a complex itinerary may need an individualized plan rather than a generic local-morning rule.

A traveler walks outside in daylight after arriving in another time zone (illustrative image)

Fixed night shifts and rotating rosters need different aims

The 2026 AASM guideline abstract conditionally suggests bright light during a night shift, a clockwise rather than counterclockwise rotating schedule, and a nap before a night shift for adults with excessive sleepiness from shift work disorder. The evidence for each of those recommendations is rated very low certainty, and the abstract warns that sleep inertia after a nap can temporarily impair performance and raise accident risk.

The same guideline says full circadian adaptation is unrealistic for rotating or frequently changing shifts. Its conditional combinations of bright light, fixed daytime sleep and reduced-light-transmittance glasses apply to adults seeking adaptation to night work, not to every worker moving repeatedly between day and night duty.

NIOSH training for nurses says a dark bedroom can support daytime sleep, but warns that dark glasses after a night shift may increase drowsy-driving risk and gives the safest use as a passenger while someone else drives. Visibility and safe transport take priority during the commute; blocking morning light is not a reason to drive while sleepy.

Melatonin and light boxes require individualized timing

The CDC says melatonin taken at the wrong biological time can increase circadian misalignment, and its useful timing changes with travel direction. This guide therefore does not provide a dose or product schedule; availability, formulation and regulatory status vary by country.

The 2026 AASM abstract makes only conditional recommendations for melatonin in adults with shift work disorder and warns that temporary sleepiness can reduce cognitive performance and raise accident risk. Any decision should be made with a clinician or pharmacist who can review the itinerary, work duties and other medicines.

Children, pregnancy and chronic illness need separate decisions

The AASM recommendations cited here concern adults with shift work disorder, and the CDC travel chapter does not set separate light or melatonin schedules for children or pregnancy. Children, pregnant people, people with chronic illness and anyone taking regular medicines should speak with a clinician before deliberately shifting sleep, using a light box or taking melatonin.

The NHLBI says light therapy can cause agitation, eye strain, headache, migraine or nausea and advises medical review before use when a person has an eye condition or takes medicine that increases light sensitivity. It also advises discussing melatonin and possible medicine interactions with a clinician, especially during pregnancy or when trying to become pregnant.

How this plays out outside Taiwan

The practical question is not only when to seek light. It is also what a worker can ask an employer to change, what a traveler can check before departure, and which rules apply to the roster. Those answers are country-specific. The examples below show why a sleep schedule cannot be separated from workplace safety or local regulation.

In the United Kingdom, the government defines a night period as 11 p.m. to 6 a.m. unless the worker and employer agree in writing to another seven-hour period that includes midnight to 5 a.m. A person who regularly works at least three hours in that period is treated as a night worker. The UK government says night workers must not work more than an average of eight hours in a 24-hour period, normally averaged over 17 weeks, and workers cannot opt out of that night-work limit. Regular overtime counts in the average, although occasional overtime does not.

That rule does not prescribe a light box, a bedtime or a melatonin plan. It sets a ceiling on exposure to night work, while the individual timing of light still depends on the roster and the worker’s biological phase. The same UK guidance says additional restrictions apply when night work involves special hazards or substantial mental or physical strain, and the employer must assess those risks. A worker should therefore ask the employer or the UK Health and Safety Executive how the organization records night hours, handles hazardous duties and provides rest, rather than treating a light schedule as a substitute for fatigue controls.

The UK government also says employers must offer a free health assessment before a person becomes a night worker. If a health professional confirms that night work has affected the worker’s health, the employer must, where possible, offer other suitable work. This is a route for discussing a persistent problem with a clinician and employer; it is not a diagnosis of shift work disorder and does not establish that a particular light exposure will work.

Singapore uses a different administrative route. A 2025 written answer from Singapore’s Ministry of Manpower says the ministry does not track how many workers have shift work sleep disorder because the condition is not listed in International Labour Organization guidelines and is not legislated in most jurisdictions, including Singapore. The answer points employers toward fatigue-risk management, job accommodations and work-environment measures that support alertness. That means a worker in Singapore who is repeatedly too sleepy for a safety-sensitive task should raise the fatigue risk through workplace channels and seek clinical advice; there is no official national count that can be used to estimate the person’s individual risk.

Singapore’s tripartite Fitness for Work guide tells employers to identify risks associated with working at night and, where reasonably practicable, consider rescheduling work to a day shift. It also treats fatigue as relevant to incident and near-miss investigations. For a traveler or worker, the useful local check is therefore the employer’s fatigue procedure and the Ministry of Manpower’s current workplace-safety guidance, not a claim that one country’s morning-light rule applies everywhere.

These examples leave two limits clear. UK working-time protections do not provide a universal sleep prescription, and Singapore’s official material does not supply a prevalence estimate for shift work sleep disorder. Before changing a roster, using a light box or taking melatonin, check the rules and clinical advice that apply in the country where the work or travel occurs.

Frequently asked questions

Does morning sunlight always move sleep earlier?
No. The CDC describes an advancing effect after the circadian low, while evening light generally delays the clock; a local morning can fall at the wrong biological time after a large time-zone change.

What is the basic difference between eastbound and westbound travel?
Eastbound travel usually calls for an earlier sleep-wake phase, while westbound travel usually calls for a later one. The route and number of time zones still determine the useful timing of light and darkness.

Should a night worker wear dark glasses while driving home?
NIOSH warns that darker glasses can compound drowsy-driving risk. Its nurse-training page describes their safest use when someone else is driving.

Can a rotating roster be solved by fully adapting to nights?
The 2026 AASM abstract says full adaptation is not realistic for rotating or frequently changing shifts. Employer scheduling, safe transport and adequate sleep opportunities remain part of fatigue control.

When is clinical assessment appropriate?
Assessment is appropriate when fatigue persists despite adequate sleep, sleep episodes occur unintentionally, or insomnia and sleepiness repeatedly impair work or daily function. Immediate safety comes first when lane drift, microsleeps or loss of alertness occurs during driving or hazardous work.