Winter light exposure, melatonin, and sleep quality

As we step into autumn and the light begins to shift, consider this a gentle lead-in to winter not far away. Winter doesn’t just change the weather; it changes your biology, as shorter days, dim indoor light, and bright screens at night quietly rewrite when you feel sleepy, when you wake, and how your sleep unfolds. If your nights start to feel different as winter approaches, there’s a circadian reason.

Table of Contents:

1. How Winter Light Rewrites the Night

2. What winter really changes

3. Clinician playbook for winter

   • Light timing and dose

   • Monitoring and testing

4. Why this matters in practice

5. Key takeaways

How Winter Light Rewrites the Night

Each winter I hear the same refrain in clinic: heavier eyelids at dinner, a second wind near 22:00, earlier wakes than planned, and sleep that feels different even when total time in bed has not changed (Mattingly S. et al., 2021). I came across a year-long wearable study that helped me connect these stories to day length itself, since longer days were linked with earlier wake times and slightly shorter sleep, which casts winter as the relative baseline for longer sleep in adults (Mattingly S. et al., 2021). At the same time, laboratory polysomnography shows a seasonal fingerprint inside the night, with more REM sleep in winter, less REM in spring and summer, and shorter REM latency in winter and early autumn, patterns that likely reflect changes in darkness signaling and melatonin timing across the year (Seidler A. et al., 2023).

What winter really changes

Light is the lead zeitgeber for the circadian system, so shorter photoperiods and cooler temperatures shift the internal map of sleep and wake, even in people who live with artificial lighting and heating (Mattingly S. et al., 2021). The seasonal signal is modest at the macro level, for example minutes rather than hours, yet it is consistent enough to appear when sleep is tracked continuously across seasons in working adults (Mattingly S. et al., 2021). At the micro level, winter tends to reshape architecture, with REM occupying a larger share and arriving sooner, while slow wave sleep shows its own seasonal pattern, including a notable dip in autumn in one clinic cohort, details that clinicians should recognize when interpreting studies ordered in different months (Seidler A. et al., 2023).

There is a second, very practical lever that often gets missed. In urban winter, habitual daytime light at the eyes is astonishingly low, with people spending about 70 percent of the day below 80 lux and only about 36 minutes above 500 lux, a range far dimmer than natural daylight outdoors, and this low midday light correlated with shorter REM latency that night and an earlier concentration of REM across the sleep period (Nowozin C. et al., 2025). These findings support a simple clinical idea: winter sleep quality depends not only on what happens after sunset, it also depends on how much light reaches the eyes before lunch (Nowozin C. et al., 2025).

SAD light essentials: intensity, spectrum, safety

A good SAD and circadian light device delivers enough biologically effective light safely and comfortably. Prioritize intensity that reaches about 10,000 lux at a practical distance or a design that provides a strong circadian stimulus, with spectra that engage the blue sensitive pathway, a wide and uniform field with low glare, clear guidance on distance, and safety features like UV blocking, low flicker, and manageable heat. Clinical validation, timers for precise morning use and evening avoidance, and reliable certifications and warranty matter. Remember that not just lux but circadian stimulus counts, since lights that target the non visual system can achieve similar effects at lower lux if comfort is maintained. 

Classic options include 10,000 lux light boxes, validated LED panels such as Litebook in trials, gentler dawn simulators that raise wakefulness though they are weaker than full boxes, and smart tunable room lighting that brightens and cools in the morning and warms and dims at night. To choose, define your goal such as seasonal mood support or sleep timing, set minimum specs, check comfort and fit on your desk, verify reviews and service in your region, and ensure CE marking and solid warranty if you are in Europe.

Clinician playbook for winter

Light timing and dose

• Morning anchor: 30 to 45 minutes of outdoor light within 60 minutes of waking, or a bright-light setup aimed toward the eyes during desk work when going outside isn’t feasible, on overcast or very dark mornings, use a SAD lamp that delivers about 10,000 lux at a practical distance (per device guidance) for 20–30 minutes with eyes open but not staring, and repeat daily to reinforce timing. s (Nowozin C. et al., 2025).

• Midday booster: A deliberate 20 to 30 minute bright-light break between 11:00 and 13:00, since lower midday light was associated with shorter REM latency and earlier REM distribution that night (Nowozin C. et al., 2025).

• Evening dimming: After sunset, keep lighting low, warm, and indirect, and reduce screen luminance, because REM timing and latency across seasons track with strength of darkness signaling and melatonin onset (Seidler A. et al., 2023).

• Weekend consistency: Preserve the morning light anchor on weekends, which stabilizes the wake time that tends to shift with changing day length through the year (Mattingly S. et al., 2021).

Monitoring and testing

• Lux audit: Run a 3 to 5 day light log labeled by morning, midday, and afternoon to quantify whether the patient is living in what amounts to biological darkness during winter days, then target increases at the times that map to healthier REM timing at night (Nowozin C. et al., 2025).

• Sleep architecture awareness: When a study is scheduled in different seasons, interpret REM percentage and latency with seasonality in mind, since winter recordings can show more REM and shorter REM latency compared with spring and summer (Seidler A. et al., 2023).

• Outcome tracking: Expect macro changes to be small but meaningful, for example a few minutes in wake time across seasons, while architecture shifts can feel larger to the patient because they change how the night is experienced rather than only how long it is (Mattingly S. et al., 2021).

Why this matters in practice

Seasonal effects are not simply about more or less sleep, they change when key stages appear, especially REM, and how easily the night stays consolidated, and these patterns are shaped by both environmental day length and the light history of the day, which gives us a concrete daytime target for winter care plans (Mattingly S. et al., 2021; Seidler A. et al., 2023; Nowozin C. et al., 2025).

Key takeaways

• Expect earlier wakes and slightly shorter sleep as day length increases, with winter acting as the comparative longer-sleep baseline in adults, and plan scheduling advice accordingly (Mattingly S. et al., 2021).

• Recognize that winter sleep often carries more REM and shorter REM latency, which can influence dream recall, morning inertia, and subjective sleep quality, and should be considered when comparing studies across months (Seidler A. et al., 2023).

• Treat midday light as a clinical tool in winter, since even within low indoor ranges, more midday light is linked to healthier REM timing that night (Nowozin C. et al., 2025).

Conclusion

Winter light doesn’t just shorten your days; it reshapes when you get sleepy, when you wake, and how your sleep stages unfold. Treat daylight like a therapeutic input: get morning light, add a midday bright break, and keep evenings dim to support melatonin and steadier nights. If this resonates, share it with someone who feels “off” every winter and wants a simple plan.

References:

1. Mattingly S., Grover T., Martinez G., Aledavood T., Robles-Granda P., Nies K., Striegel A., Mark G. (2021). The effects of seasons and weather on sleep patterns measured through longitudinal multimodal sensing. Nature. https://doi.org/10.1038/s41746-021-00435-2

2. Seidler A., Weihrich K.S., Bes F., de Zeeuw J., Kunz D. (2023). Seasonality of human sleep: Polysomnographic data of a neuropsychiatric sleep clinic. Frontiers. https://doi.org/10.3389/fnins.2023.1105233

3. Nowozin C., Wahnschaffe A., de Zeeuw J., Papakonstantinou A., Hädel S., Rodenbeck A., Bes F., Kunz D. (2025). Living in Biological Darkness II: Impact of Winter Habitual Daytime Light on Night‐Time Sleep. European Journal of Neuroscience. https://doi.org/10.1111/ejn.16647