Light and seasons

Day length

Seasonal changes are the consequence of the rotation of the Earth around the sun and the tilt of the Earth’s axis relative to its orbital plane. A clear change that we can observe as seasons passes (specifically as we move further from the Equator), is that the length that daylight is available changes. Summer time is characterized by long days and short nights while wintertime shows short days and long nights. Many species respond to these fluctuations in day-length and adapt the timing of their breeding, moult or migration (birds) according to the appropriate season. Humans are less photoperiodic, that means having little fluctuations in their behavior changing over the season. This is at least true in the current society, but a clear annual rhythm in for example conception and birth was seen before the industrial revolution, with a peak of conceptions in Europe at the end of spring (Foster & Roenneberg 2008). Nowadays an annual rhythm in conceptions is hardly observed anymore, and one can speculate whether the widespread use of artificial light inducing a less clear seasonal change in day-length in humans plays a role. Nevertheless, although perhaps less visible in daily life, some physiological assessments show that there are seasonal patterns in for instance the concentration of the neuropeptide hypocretin in the brain related to day length in humans (Boddum e.a. 2016).


Seasonal variation in mood and sleep

If seasonal changes in human behavior occur, for instance in sleep and wellbeing, they are specifically observed in countries that have very short days in winter and long days in summer. In general, humans sleep slightly later and sometimes a bit longer in winter compared to summer; sleep duration is on average 1.5 hours longer in winter in people who experience a clear change of seasons (Kasper et al 1989). These people also report lower levels of daytime energy, and mood changes such as a higher level of irritability in winter than in summer. In addition they often experience an increase in appetite with a preference for carbohydrates, and an increase in weight of 2-5 kg in winter compared to summer. Symptoms often start in autumn, with November, December and January being the months mentioned as worst. In a small part of the population, the symptoms become more severe, and we speak from Seasonal Affective Disorder, winter type, or a “winter depression” (Mersch et al 1999).

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