You know the feeling. You stir in the dark, head buzzing first with thoughts about the day ahead, thoughts which soon yield to despair about getting through it after rest has been cut brutally short. Such anxiety may seem part of the human condition, but in fact it's a poisonous product of the relatively recent fluorescent turn taken by western societies. Everybody knows the sleep cycle runs over four hours, so why do we panic at failing to forcibly bolt two cycles together into an eight-hour slumber? Craig Koslofsky, a historian who illuminated the whole idea of the night on a recent World Service broadcast, has established that in pre-street-lit days the custom was to rise after just four hours – not for fretful wakefulness, but for prayer, talk or indeed sex – before a second sleep. "Regenerate man finds no time so fit to raise his soul to Heaven, as when he awakes at mid-night," wrote one scribe of "Mid-Night Thoughts" in 1682. Would that it still were so.