Late circadian and sleep timing are associated with negative cognitive performance and health outcomes such as daytime sleepiness, reduced driving and school performance, substance abuse, mood disorders, diabetes, and obesity. Contributing factors to late sleep timing include the period and phase of the circadian clock, exposure to light at night, circadian rhythm sleep-wake disorders, stimulant intake, and work/school week versus weekend social schedules. Researches have shown later circadian and sleep timing occur after exposure to the modern electrical lighting environment compared to the natural summer light-dark cycle.
The research findings demonstrate that the human melatonin rhythm adapts to short summer and long winter nights when living in a natural light-dark cycle. It further shows that living in the modern electrical lighting environment reduces seasonal circadian responsiveness by delaying the beginning of the biological night in both winter and summer. It has been argued that humans live in a constant summer photoperiod (i.e., duration of light exposure) in the modern electrical environment, and this appears to be true for the duration of biological night but not for circadian timing. Modern lighting environmental conditions do not entirely replicate living in the natural summer photoperiod. Researchers observed that the timing of the middle of the biological night, but not the middle of the sleep episode, occurs close to the timing of the middle of solar darkness when living in natural winter and summer light-dark cycles, but less so when living in the modern environment. A weekend of camping prevents the typical weekend circadian and sleep delay, which is an important contributor to the phenomenon of social jet lag. Specifically, the weekend phase delay in the modern electrical lighting environment contributes to social jet lag on Monday morning since there is a mismatch between biological (circadian delay) and social (awakening early for work/school) timing, the definition of social jetlag. This suggests that weekend exposure to the natural light-dark cycle may help with social jet lag, and also with initiating treatment for winter depression, and circadian rhythm sleep-wake disorders (e.g., delayed sleep-wake phase) that show late sleep and/or circadian timing
Besides light abundance, the spectral composition of light is also important since our circadian system is more sensitive to shorter wavelength (blue) light than longer wavelength light, which has been shown to vary with seasons.
A UK-based study using wrist-worn light meters observed that not only was total light exposure lower in winter than in summer, but also daily blue light exposure in summer was almost double than that in winter.
The seasonal differences in abundance and composition of light exposure were most pronounced during the evening hours, when light exposure would be expected to induce phase delays in the circadian system. These differences in light exposure would affect the phase of entrainment of the circadian clock differently in summer compared to winter and thus potentially affect chronotype in a seasonal manner.