What February does to children's sleep, and what to do about it.

The circadian rhythm — the internal clock that regulates sleep and waking — is anchored primarily by light, specifically by the blue-wavelength light that the human eye is most sensitive to. In February in Vienna, or London, or Edinburgh, the available natural light is at its lowest point of the year. The days are short. The light that exists is low-angle and filtered.

The result, for many children, is a circadian clock running without adequate anchoring. Sleep onset shifts later because the evening signal — the reduction in light that tells the circadian system that night is approaching — is arriving late against an indoor background that is already low-light. The morning waking signal — the light that should anchor the day — is often absent because children wake before sunrise or in light conditions too weak to register.

This is not a behaviour problem. It is a photobiology problem. The child who won't fall asleep in February and can't wake in the morning is not being difficult. Their circadian system is running unanchored because the light environment has not provided the information it needs. In the Vienna clinics I run, I see this expressed as: the child who cannot eat breakfast, cannot sit still, cannot manage sibling interactions — all symptoms of the same circadian desynchrony.