The timing glitch: why it happens at 5am and after a lie-in
REM periods get longer towards morning. More REM means more chances to wake up inside one.
Quick facts
Typical length20 seconds - 2 minutes
Ends by itselfAlways, without exception
Physical harmNone documented
People affectedAbout 8% at least once
A sleep cycle lasts about 90 minutes. Early in the night it is dominated by deep sleep; towards morning REM takes over, and the last REM block can last 30 to 45 minutes.
Key idea
If something wakes you during that long morning REM block - light, an alarm, a partner, a full bladder - you have a much higher chance of surfacing while atonia is still active.
This is why episodes cluster on weekends and holidays: sleeping in adds extra REM at exactly the hours where you are most likely to half-wake.
Irregular schedules, shift work and jet lag do the same thing by pushing REM into unusual moments.
Morning REM blocks are the longest.
Lie-ins add REM, not danger.
A stable wake time reduces episodes.
Sources & further reading
Links to peer-reviewed research and recognised medical organisations.