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A promising finding

Bad sleep raises evening cortisol, and evening cortisol predicts sleep

One or two curtailed nights lifted evening cortisol by 30 to 45% and flattened the day's whole curve. Across 15 days, a higher bedtime reading came before a shorter, less efficient night.

Cortisol

A bad night gets charged to the next day and left there. In small experiments, one or two short nights lifted evening cortisol by 30 to 45% and flattened the day's curve, and nobody reported feeling more stressed. In 95 young adults tracked for 15 days, a high bedtime cortisol reading was followed by a night that was shorter and less efficient. Both halves are small and young, 13 men in one experiment, and the longer study only watched. The cost of a bad night is not only the day after it.

The findings behind this

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SleepA night or two of short sleep raised evening cortisol by 30 to 45%

In small experiments, one or two curtailed nights delayed the evening fall by at least an hour and flattened the whole day's curve. Nobody reported feeling more stressed.

★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.These were tiny experiments, 13 men in one of them, run over one or two nights. All the participants were young men, so nothing here covers women or older adults. And a night or two of lost sleep is not years of it, so how far the flattening goes over a lifetime is unmeasured.Click to keep this openSleep betterCortisol
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SleepCortisol before bed predicted the night, and the night the next day

Across 15 days in 95 young adults, a higher cortisol reading before bed came before a shorter, less efficient night, and the shortest sleepers had a flatter curve the next day.

★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.The authors describe the effects as small. Everyone measured was young and healthy, at an average age of 20, so this says nothing about older adults or about anyone with a sleep or stress disorder. And these are the same people watched over time rather than an experiment, so something else in their days could be driving both ends.Click to keep this openSleep betterCortisol

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