midsleep

Midsleep is the midpoint of a sleep period, defined as the time halfway between sleep onset and awakening. It is most often used to describe the timing of the main sleep episode, usually overnight, though it can refer to a primary sleep block in polyphasic schedules. Midsleep is calculated by adding half of the total sleep duration to the sleep onset time. For example, sleep on at 11:00 PM and wake at 7:00 AM yields a midsleep of 3:00 AM.

In chronobiology, the midsleep time serves as a simple proxy for chronotype. An earlier midsleep is associated with morningness, while a later midsleep aligns with eveningness. Researchers sometimes report midsleep on free days (MSF) as an estimate of an individual’s intrinsic sleep timing, and MSF corrected for sleep debt on workdays (MSFsc) to separate circadian preference from social constraints. These measures are used in studying social jet lag and circadian alignment in populations.

Measurement methods include sleep diaries, actigraphy, and polysomnography. Each method estimates sleep onset and offset, from which the midsleep is derived. Limitations include reliance on accurate reporting or device detection, potential biases in irregular or napping-heavy schedules, and the challenge of applying a single midsleep value to multifaceted sleep patterns.

Midsleep is a widely used, noninvasive metric in sleep and circadian research, aiding in the characterization of individual timing, assessing alignment with societal schedules, and tracking changes due to aging, shift work, or treatment of circadian rhythm disorders.