The autumn equinox and children's biological rhythm.

Light is the primary regulator of the human circadian system. The circadian system — the biological clock — sets itself against the dawn signal: the first light of day reaching the retina tells the body what time it is, which triggers a cascade of hormonal responses that set the rhythm of the day: cortisol rises for alertness, melatonin will fall for wakefulness, and the system programmes when melatonin will rise again in the evening for sleep.

In summer, the dawn signal arrives early — sometimes before 5am — and the biological clock sets itself accordingly. Children wake more easily because their biology has already prepared them. In autumn, the dawn signal arrives progressively later. The same child who woke easily at 6:30am in July is now being asked to wake at the same time, but their biology hasn't yet received the light signal that tells them morning has arrived.

This is not laziness. This is a mismatch between the biological clock and the social clock — school start times don't adjust for season, but children's physiology does.

In Vienna we see this in clinic every September without exception. The same children who were easy to wake in July are difficult to wake in October. The parents report it as a change in the child. It is a change in the light.