In athletes with high levels of the stress hormone cortisol, there is a measurable link to worse sleep quality, as determined by a standard sleep questionnaire.

Athletes who sleep poorly are more likely to have elevated cortisol levels than those who sleep well, even when accounting for differences in sport, gender, and age. This suggests cortisol levels may be a stronger indicator of stress-related physiology than these demographic factors.

In athletes undergoing intense training, differences in sleep quality are not clearly linked to whether they compete in athletics or other sports once you account for their cortisol levels, suggesting that overall training stress may matter more than the specific sport.

Among athletes with intense training schedules, differences in sleep quality are not reliably linked to whether someone is male or female, or to their age, once cortisol levels are taken into account.

In elite athletes training for national competitions, a blood cortisol level of 208.05 nmol/L or higher is associated with a 75% chance of correctly identifying poor sleep quality and a 71.9% chance of correctly identifying good sleep quality, suggesting it could be used as a biological indicator to detect sleep disruption.

Not getting enough sleep can hurt athletic performance, especially for sports that need quick thinking or precise moves—like tennis or basketball—more than for sports that are mostly about strength or speed, like sprinting.