Sleep doesn't repair you. Your circadian clock does. Sleep is merely the key.

Most sleep advice stops at duration. Eight hours and you're done. But the biology is more layered than that and a large study published in SLEEP (2024) makes it hard to argue otherwise.
https://pubmed.ncbi.nlm.nih.gov/37738616/

Windred et al. tracked over 60,000 people using objective accelerometer data for up to 7.8 years. The finding that should rewrite how we counsel patients: sleep regularity was a stronger predictor of all-cause mortality than sleep duration. The most irregular sleepers carried a 20 to 48% higher mortality risk. For cancer mortality specifically, irregular sleep was a significant predictor while short sleep duration was not. That last point deserves a moment of attention.

The mechanism runs through circadian disruption. Sleep is not passive, it actively drives repair through slow-wave GH secretion, glymphatic clearance, and hypothalamic regeneration. But your circadian clock sequences when those processes are maximally available. Sleep at the right phase and you get full biological yield. Sleep at a drifting or irregular phase and the same hours produce measurably less output. Physical regeneration is particularly sensitive to this, night shift workers demonstrate the growth hormone axis underdelivering regardless of total sleep time.

Duration is the floor. Regularity is the ceiling.

The practical translation from this study: people in the top 20% of sleep regularity fell asleep and woke within roughly one-hour windows most days. The bottom 20% operated within three-hour windows.
That gap, not their total hours, predicted who lived longer.

If you sleep "enough" but still feel depleted, the first question is not how much. It is when, and how consistent is the when.

Regularity is the lever many people aren't pulling. The data says it may be the most important one.