Sleep timing may modulate the effect of sleep loss on testosterone.

Plain-language summary

This study asked whether it matters which half of the night you lose sleep from, and it is two experiments rather than one. In the first, fifteen young healthy men slept four hours from 02:45 to 07:00 for two nights, which cuts the first half of the night and keeps the second, and their hormones were tracked across the following fifteen hours. Nothing differed from a normal eight hour night. In the second, eight men were measured on the morning after three different nights, one with no sleep at all, one of four and a half hours from 22:30 to 03:30 which keeps the first half and cuts the second, and a normal seven hour night. Both the sleepless night and the early night lowered morning testosterone and prolactin (a pituitary hormone that suppresses testosterone when it runs high, so lower is not automatically better).

Key findings

• Fifteen men who slept four hours from 02:45 to 07:00, losing the first half of the night, showed no significant change in testosterone or prolactin against a normal eight hour night.
• Eight men who slept four and a half hours from 22:30 to 03:30, losing the second half of the night, had lower morning testosterone and prolactin.
• A night with no sleep at all also lowered morning testosterone and prolactin in those eight men.
• The two results come from two separate experiments with different men, fifteen in one and eight in the other.