A 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.
What the study found
Two short nights lifted evening cortisol by 30%
Cortisol normally falls away through the evening to a low point at night. One or two nights of curtailed or missing sleep delayed that fall by at least an hour and lifted evening cortisol by 30 to 45%, flattening the day's whole curve. Nobody reported feeling more stressed.
The caveat · Tiny trials, as few as 13 men
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.