A long weekend lie-in does not fully undo a week of short nights. Sleep is not a single quantity being topped up, and its internal structure explains why recovery is partial.
Sleep is organised into cycles
A night's sleep runs through repeated cycles, each moving from lighter stages into deep slow-wave sleep and then into rapid eye movement sleep.
The proportions shift across the night, with deep sleep concentrated in the early cycles and rapid eye movement sleep lengthening toward morning.
Shortening a night therefore does not remove sleep evenly. It removes disproportionately from whichever stages sit at the end.
Different stages support different processes
Slow-wave sleep is associated with physical restoration and with the consolidation of certain kinds of memory.
Rapid eye movement sleep is linked to different consolidation processes and to emotional regulation, and it is the stage most reduced by a late night followed by an early start.
Because the stages are not interchangeable, extending one night does not straightforwardly replace what was lost from another.
Recovery sleep is compressed rather than extended
After restriction, the body prioritises deep sleep in the following night, entering it faster and spending longer there.
This rebound is efficient but limited, since there is only so much deep sleep a single night can accommodate.
Rapid eye movement rebound tends to appear later, over subsequent nights, which is why full recovery from a sustained deficit takes longer than the deficit itself.
Timing interacts with the internal clock
Sleep is governed both by accumulated pressure and by a circadian rhythm that determines when sleep comes easily and when it does not.
Sleeping much later at weekends shifts that rhythm, so the following week begins with the internal clock misaligned to the required schedule.
The recovery sleep and the schedule shift therefore work against each other, which is why the benefit of a long weekend often does not survive Monday.
What the research does not settle
Studies measure recovery of alertness and performance, and these return toward baseline faster than some underlying physiological markers do.
Feeling recovered is therefore not strong evidence that everything has recovered, which complicates any judgement made from experience alone.
Persistent difficulty with sleep has many possible causes, several of them treatable, and it is a matter to raise with a qualified clinician rather than to manage by schedule alone.