Sleep glossary
Sleep fragmentation
When sleep is repeatedly interrupted by brief arousals you never remember, why the total can look fine while the night is not, and how it is detected.
In short
Sleep fragmentation is sleep repeatedly broken by brief arousals, most of which you never remember. Total sleep time can look normal while the night delivers much less than it should, because each interruption resets the descent through the stages.
Sleep fragmentation is also called fragmented sleep or sleep disruption.
Fragmentation is about the shape of a night rather than its length. Sleep is repeatedly broken by arousals, brief shifts toward wakefulness that may last only seconds, and the critical feature is that most of them never reach the threshold at which you would form a memory. The night is remembered as continuous. It was not.
The consequence is that duration stops being a reliable proxy for how much sleep you got. Every interruption pushes you back toward the lighter stages, so a heavily fragmented eight hours can contain considerably less deep sleep than an unbroken seven. This is the most common explanation for the complaint that the clock says a full night and the morning says otherwise.
How it is actually detected
This is the measurement most people cannot do from memory, and it is also the one where the difference between instruments matters most. A polysomnogram scores arousals from brain activity. A wrist device infers them from movement. A phone on a nightstand works from movement and sound, which detects the larger disturbances and misses the smallest ones.
That is a real limitation and it is worth stating rather than glossing: a phone will underestimate fragmentation. It will also, reliably, tell you more than remembering the night will, which is the honest case for it. Sleep tracking without a wearable sets out where the line sits.
The related figures are WASO, which totals the time awake after first falling asleep, and sleep efficiency, which is the proportion of time in bed actually spent asleep. Fragmentation can drag both down while total sleep time looks unremarkable.
Persistent fragmentation with no obvious external cause, especially with snoring or daytime sleepiness, is a clinical question. Tock does not detect, screen for or suggest sleep apnea, and it is not a medical device.
Questions people ask
Can you have fragmented sleep without knowing?
Routinely. An arousal has to last somewhere in the region of a minute or more before most people form a memory of it, so a night with dozens of brief interruptions can be remembered as unbroken. That gap between the felt night and the actual one is the whole difficulty.
Why do I feel tired after eight hours of sleep?
Fragmentation is one of the common explanations. Eight hours interrupted forty times is not eight hours of sleep in any functional sense, because each interruption sends you back toward the shallow stages, so the night contains far less deep sleep than the total implies.