The days are getting shorter by a few minutes a day, like every year, and the body notices it immediately. The credit goes to the biological clock, which follows the light and not the calendar: every morning a small group of neurons in the hypothalamus (the suprachiasmatic nucleus) receives information from the retina on how much light there is around and consequently regulates the release of melatonin, the hormone that prepares for sleep. When this light begins to run out, as happens in autumn, the entire system changes.
The internal clock responds to light, not to the seasons on the calendar
A review published in Somnologie clearly reconstructs this mechanism: natural light, when exposure is sufficient, brings forward sleep times, modifies its duration and improves its quality. With fewer hours of daylight, melatonin secretion is prolonged, the biological night becomes longer, and sleep tends to increase even with the same daily commitments.
Less sun, more tiredness: what really changes in sleep
The phenomenon does not require extreme latitudes to manifest itself, the transition from summer to already late autumn is enough for the sleep phase to move forward. Some studies cited in the same review show that anyone who lives with limited natural exposure, perhaps because they spend daytime hours in the office, has a less synchronized sleep-wake rhythm than people who spend more time outdoors. The difference is not so much in the quantity of sleep, but in its location and the quality perceived upon awakening.
The link with mood: from melatonin to serotonin
Here a second protagonist comes into play, serotonin, whose balance is in turn affected by less sun exposure. It is the mechanism underlying seasonal affective disorder (SAD), described for the first time in 1984 by the psychiatrist Norman Rosenthal and today recognized by the National Institute of Mental Health as a form of recurrent depression, linked to the hours of light available rather than to the calendar. Be careful, going through a slower than usual autumn does not mean suffering from SAD in the clinical sense, and most people only experience an attenuated version, that sense of tiredness which in English is called winter blues.
What really makes the difference
Research on phototherapy, summarized in a review published in BMC Psychiatry, indicates that exposure to bright light early in the day produces a range of measurable benefits on seasonal depressive symptoms, often within a few weeks. A morning walk, when the sky is still clear, guarantees comparable exposure, so it is truly within everyone’s reach. Consistency counts, researchers argue, more than the intensity of a single moment.
