The two nurses Professor Jae Kyoung Kim was studying were alike in many ways.

They both worked odd hours and, over the course of two weeks, slept for a similar amount of time. Yet, one of the nurses was getting sufficient sleep less than 50 per cent of the time while the other was getting enough more than 90 per cent.

Why?

The amount of sleep we need to feel alert each day strongly depends not just on the average time we’ve slept. Rather it’s influenced by two processes: our sleep duration plus whether the time we go to sleep and wake up aligns with our circadian rhythm.

When we close our eyes at night and drift off, sleep pressure – the need to go to sleep – slowly decreases. When it drops below the rising of our circadian rhythm, we wake up.

Then, while we are awake during the day,

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