Sleep is often discussed as a nighttime problem. In reality, sleep begins the moment you wake up.

A growing body of circadian biology research shows that when you are exposed to sunlight matters as much as how long you sleep. A recent 2025 study published in BMC Public Health, titled “The role of sunlight in sleep regulation: analysis of morning, evening and late exposure”, reinforces a crucial message for modern lifestyles:
sunlight is the master regulator of human sleep timing.

This article explores the science behind sunlight and sleep, why morning light is uniquely powerful, how evening light disrupts sleep, and how these insights are increasingly relevant in psychiatry, sleep medicine, and everyday mental health.

Sleep is governed by biology, not just habits

Human sleep is controlled by two interacting systems:

1. Sleep homeostasis – the pressure to sleep that builds with wakefulness

2. The circadian rhythm – an internal 24-hour clock that decides when sleep should occur

While sleep pressure responds to fatigue, the circadian clock responds primarily to light—specifically, natural sunlight.

This clock is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is directly connected to the eyes through specialized retinal cells sensitive to light, especially blue-wavelength light present in sunlight.

Light exposure resets this clock every single day.

What the 2025 BMC Public Health study adds

The BMC Public Health study analysed patterns of sunlight exposure at different times of day and correlated them with sleep timing and quality.

Key findings:

• Morning sunlight exposure was associated with:

  • Earlier sleep onset

  • Better sleep regularity

  • Improved subjective sleep quality

• Evening and late-day sunlight exposure was associated with:

  • Delayed sleep onset

  • Circadian phase delay

  • Greater risk of sleep–wake misalignment

This aligns with decades of circadian research:
👉 Light early advances the biological clock
👉 Light late delays it

Why morning sunlight is uniquely powerful

Morning sunlight does three critical things:

1. Suppresses melatonin at the right time

Melatonin is not a “sleep hormone” alone—it is a darkness signal. Morning light shuts it off decisively, marking the start of the biological day.

2. Starts the cortisol rhythm

Healthy cortisol release peaks in the morning. Morning sunlight strengthens this rhythm, improving alertness, mood, and energy.

3. Sets a countdown timer for sleep

Once the clock is set in the morning, melatonin release in the evening occurs predictably—about 14–16 hours later.

Without sufficient morning light, this entire rhythm drifts later.

Evening light: the silent sleep disruptor

Evening and late-day light exposure—whether from the sun, indoor lighting, or screens—sends a conflicting signal to the brain: “Daytime is still ongoing.”

This results in:

• Delayed melatonin secretion

• Increased alertness at night

• Difficulty falling asleep despite fatigue

Importantly, even sunlight in the early evening can delay circadian timing, especially in people already prone to delayed sleep phase (common in adolescents, ADHD, and mood disorders).

Circadian misalignment and mental health

Sleep disruption is not just inconvenient—it is biologically destabilising.

Circadian rhythm disturbances are strongly linked to:

• Depression

• Bipolar disorder

• Anxiety disorders

• ADHD

• Metabolic syndrome

Large-scale studies show that circadian misalignment worsens emotional regulation, impulse control, and cognitive performance—often mimicking or amplifying psychiatric symptoms.

In many cases, patients labelled as having “insomnia” are actually suffering from circadian delay, not lack of sleep drive.

Why modern lifestyles are biologically hostile to sleep

Compared to our ancestors, modern humans experience:

• Less daylight exposure overall

• Almost no bright light in the morning

• Excessive light at night

Indoor lighting rarely exceeds 300–500 lux, whereas outdoor daylight often exceeds 10,000 lux even on cloudy days.

The result: a weak daytime signal and a strong nighttime signal—the worst possible combination for sleep.

Clinical implications: light as treatment, not advice

Modern sleep and psychiatric practice increasingly treats light exposure as a biological intervention, not lifestyle counselling.

Evidence-based recommendations include:

• 20–30 minutes of outdoor sunlight within 1–2 hours of waking

• Avoid sunglasses during this exposure if safe

• Reduce bright and blue light after sunset

• Maintain consistent light–dark timing across weekdays and weekends

For delayed sleep phase disorders, morning light therapy is often more effective than hypnotic medications.

A shift in how we think about sleep

Sleep does not start at bedtime.
It starts with light exposure in the morning.

This reframes treatment entirely:

• Insomnia becomes a rhythm problem

• Fatigue becomes a timing issue

• Mood instability becomes a circadian vulnerability

Sometimes, the most advanced intervention is not technological—it is biological alignment with the sun.

Conclusion: returning to the original sleep regulator

Sunlight shaped human sleep for hundreds of thousands of years. Artificial light has existed for barely a century.

The growing scientific consensus—including the 2025 BMC Public Health study—is clear:
reconnecting sleep to natural light timing is one of the most powerful, underused tools in mental and physical health.

Key References

1. Menezes-Júnior LAA et al. The role of sunlight in sleep regulation: analysis of morning, evening and late exposure. BMC Public Health. 2025;25:3362.

2. Czeisler CA, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. 1999.

3. Khalsa SBS et al. A phase response curve to single bright light pulses in humans. J Physiol. 2003.

4. Wright KP et al. Entrainment of the human circadian clock to the natural light–dark cycle. Curr Biol. 2013.

5. Foster RG, Wulff K. The rhythm of rest and excess. Nat Rev Neurosci. 2005.

6. Walker MP. Why We Sleep. Scribner; 2017.

About the Author

Dr. Srinivas Rajkumar T, MD (AIIMS), DNB, MBA (BITS Pilani)
Consultant Psychiatrist & Neurofeedback Specialist
Mind & Memory Clinic, Apollo Clinic Velachery (Opp. Phoenix Mall)

Dr. Srinivas Rajkumar specialises in sleep disorders, circadian rhythm disturbances, mood disorders, ADHD, and objective neurobiological assessments. His clinical approach integrates neuroscience, behavioural rhythm alignment, and judicious medication use for sustainable mental health outcomes.

✉ srinivasaiims@gmail.com
📞 +91-8595155808