Seasonal Effects on Sleep
Seasonal variation can have an impact on our sleep/wake cycle. For example, studies have shown that during summer sleep times, core body temperature and melatonin secretion (a hormone involved in the timing of sleep) all tend to be slightly advanced. This means that they tend to occur earlier in the night relative to winter months (e.g. when studied under controlled experimental conditions, people tend to go to bed earlier and wake-up earlier in summer). The main reason for this is due to the photo-period (length of light exposure), which tends to be greater in the summer months. Light exposure early in the morning can impact our internal biological clock, shifting the timing of the sleep window.
A recently published study by Friborg et al. (2011) supports the idea of seasonal variation in sleep due to daylight duration. The study compared Norway, a country that undergoes large seasonal variation, with Ghana, a country that undergoes very little seasonal variation (due to its position close to the equator). Seasonal effects were found for Norway, with bed and rising times being earlier in summer, while insomnia, fatigue, and low mood were more prevalent in winter. These winter-summer seasonal differences were not found to be present for Ghana.
Another related and well-studied phenomenon is Seasonal Affective Disorder (SAD), this is where people experience a change in mood due to changing seasons, but who otherwise are typically in good mental health for the rest of the year. In line with the study mentioned above, the most common finding is that depressive symptoms tend to peak in winter. For patients with SAD, there is evidence that prescribed light exposure (through a so-called lightbox) can reduce depressive symptoms. The timing of light exposure is very important and should be guided by one's doctor or health professional.
Friborg, O., Bjorvatn, B., Amponsah, B., Pallesen, S. (2012). Associations between seasonal variations in day length (photoperiod), sleep timing, sleep quality and mood: a comparison between Ghana (5°) and Norway (69°). Journal of Sleep Research, 21(2), 176-184.