Maybe you’re a new parent or someone who lies awake at night. If so, you may have started to worry you’re not getting enough sleep.

Sleep wearables don’t help. They can show your “sleep debt”, a running total of how far you’ve fallen behind.

But the word “debt” assumes your sleep works like a bank account. It assumes lost hours stack up, carry over, and you must eventually repay them in full.

But sleep doesn’t really work this way. And chasing “enough sleep” may not be helping.

What is a sleep debt?

Two systems control your sleep. One is your body clock, which helps keep wakefulness and sleep aligned with the day and night. The other is the one that matters here: sleep pressure.

Sleep pressure builds the longer you stay awake and eases while you sleep. At its highest, it’s hard to resist. Someone pulling an all-nighter might find themselves nodding off unintentionally.

This biological process is what “sleep debt” is trying to describe. If you sleep less than your body needs, pressure for sleep builds. Given the opportunity to recover after lost sleep, you sleep longer. In this broad sense, the debt metaphor works.

But this metaphor has some assumptions that don’t fit with our biology.

If you have a financial debt, the maths is........

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