What we set out to study
Most previous research on seasonal sleep patterns has been limited to single countries, making it hard to separate the effect of daylight from the effect of local culture, work schedules, and social habits. We wanted to test this on a global scale.
Using a large anonymized dataset using wearable and sensor data from people across hundreds of countries we asked three core questions.
Does sleep duration actually change with daylight hours?
Does this effect get stronger the further you live from the equator (where seasonal swings in daylight are more dramatic)?
Do different countries respond to seasonal light changes differently, even after accounting for geography?
What we found
Daylight does affect sleep, but only slightly. For every extra hour of daylight in a day, people slept about ~5 fewer minutes. Over a year, the difference between the longest and shortest only shifts your sleep by about 15–20 minutes on average, depending on where you live.
Latitude barely matters. You might expect people in Scandinavia or northern Canada, where summer days can last 18+ hours, to show much bigger seasonal sleep swings than people near the equator. We found almost no evidence of this. The photoperiod effect was essentially the same whether you lived at 10° or 60° latitude.
Season labels are misleading. Many previous studies reported that people sleep differently in "winter" versus "summer." When we tested this with our global dataset, those categorical seasonal effects disappeared almost entirely. Once we accounted for actual daylight hours, labels like "spring" and "autumn" added no meaningful explanatory power.
Culture and country matter more than light. This was perhaps our most striking finding. Different countries showed very different baseline sleep durations and different sensitivities to daylight changes, even after adjusting for latitude and individual differences.
Why this matters
The practical takeaway is that your sleep patterns are shaped far more by your daily routines, cultural context, and personal habits than by the season. While daylight does exert a small, consistent pull on how long we sleep, it's dwarfed by the influence of work schedules, social norms, and lifestyle.
Link to pre-print research paper here.
