Why afternoon sunlight could be keeping your teen awake at night

Why afternoon sunlight could be keeping your teen awake at night

Do you think afternoon sunlight helps teenagers at night? New research shows it can actually disrupt their melatonin and delay sleep, which suggests school schedules, not just screen time, may be the key to better rest.

In a recent article published in the JournalNPJ Biological timing and sleepResearchers investigated whether exposure to moderate or bright afternoon to early evening (AEE) light could reduce the effects of evening light on melatonin suppression in Swiss adolescents.

Their results suggest that exposure to bright light during AEE resulted in lower levels of melatonin during subsequent exposure to evening light, with melatonin onset delayed by 21 minutes, suggesting a possible acute phase delay effect on circadian rhythms combined with increased alertness.

background

Teens are particularly vulnerable to chronic sleep deprivation, with more than half of 14 to 17-year-olds getting less sleep than recommended. This shortfall, often caused by early schooling and delayed bedtimes, can negatively impact mental health, behavior and cognitive performance.

A biological delay in circadian timing during puberty contributes to later sleep, a tendency reinforced by increased nocturnal autonomy and exposure to stimulating activities and light. Because light regulates the body's internal clock, poorly timed light, especially in the evening, can increase melatonin, circadian rhythms and alertness.

Morning light exposure is known to advance circadian timing, but many adolescents miss this opportunity due to school schedules. Meanwhile, turning off artificial lights or screens in the evening can worsen sleep problems.

Restricting evening light is often inconvenient for adolescents, leading to interest in alternative sleep management strategies. Recent research in adults suggests that exposure to bright light earlier in the day may reduce sensitivity to light later at night, potentially buffering the negative effects of evening light exposure.

However, little is known about how such effects play out in adolescents, particularly in relation to the timing and intensity of prior light exposure. This study addresses this gap by testing whether different AEE light levels can alter physiological responses to later evening light in adolescents.

About the study

This study included 22 healthy German-speaking adolescents between the ages of 14 and 17 and was conducted in Basel, Switzerland.

Each participant completed three laboratory sessions under different lighting conditions, dark, moderate and bright, in a counterbalanced order with at least a week scheduled between sessions.

Before each session, participants followed a five-day sleep-wake stabilization routine monitored via wrist actimetry and sleep diaries. Each laboratory session lasted 18.5 hours, during which participants were exposed to controlled lighting.

Saliva samples were collected to assess melatonin levels before and after light exposure. Additional measures included subjective sleepiness, vigilant attention, skin and room temperatures through a distal to proximal skin temperature gradient (DPG), and activity logs.

Bright light history was also tracked with wrist sensors during the pre-lab days.

Statistical analysis involved the use of linear mixed models to assess the effects of lighting conditions on outcomes including melatonin suppression, sleepiness and body temperature regulation, while controlling for time, light history, chronotype and age.

Key Findings

The study examined how different exposures to AEE light affected adolescents' physiological and behavioral responses to later evening light.

• Die helle Exposition von AEE -Licht verringerte die Abendmelatoninspiegel im Vergleich zu schwachem Licht signifikant, während mäßiges Licht einen nicht signifikanten Reduktionstrend in dieselbe Richtung zeigte (β = -7,37, p = 0,114).
• Das helle AEE -Licht verzögerte auch den Beginn des Melatonins um ungefähr 21 Minuten, was auf eine akute Phasenverzögerung hinweist, die potenzielle Schutzanpassungen überschreien kann.
• Bemerkenswerterweise war eine größere Exposition gegenüber hellem Licht (mehr als 1000 Lux) in den 32 Stunden vor der Laborsitzung mit höheren Melatoninspiegeln und früheren Beginn am Abend verbunden, was auf einen adaptiven Effekt der Tageslichtgeschichte hinweist.

Subjective sleepiness during late evening cross-light exposure did not differ significantly between lighting conditions. During the AEE photoperiod itself, both moderate and bright light reduced sleepiness compared to dim light. Bright light also slowed the increase in sleepiness over time.

Vigilant attention, measured by reaction speed in a psychomotor task, showed no significant changes either during the intervention or later in the evening.

For DPG, bright AEE light increased DPG both during and after exposure, while moderate light reduced it, indicating opposite thermoregulatory responses. These effects were confounded by higher room temperature during the bright light condition. The bright light story did not significantly influence DPG.

Conclusions

This study examined whether exposure to AEE light could help adolescents resist the vigilant effects of evening light. Contrary to expectations, late evening bright AEE light reduced melatonin levels and delayed melatonin onset, possibly due to an acute phase delay effect that overrides adaptive responses.

While it temporarily increased alertness, it did not improve reaction times. Notably, adolescents' earlier exposure to bright light was associated with earlier melatonin onset and higher evening melatonin levels, suggesting that long-term light history may support circadian alignment.

Although the study was limited by a small, homogeneous sample, the lack of a weak evening condition, and the confounding of room temperatures, they had several strengths: precise laboratory control, real-world lighting conditions, and high-resolution melatonin sampling. Within-subject engineering and careful monitoring of light history improved the reliability of the data.

These results highlight the complex role of light and the history of circadian physiology in adolescents. Afternoon bright light up to three hours before bedtime may be inadequate for mitigating evening light effects. Policy changes, such as Other measures, such as delaying school start times, are highlighted by the authors as potentially more effective solutions to improving sleep health in adolescents.

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