Sleep glossary
Sleep architecture
Two people can sleep the same number of hours and have completely different nights. Sleep architecture is the word for that difference.
In short
Sleep architecture is the overall structure of a night: which stages you pass through, in what order, for how long, and how often you surface. It is read off a hypnogram rather than judged from any single number.
Sleep architecture is also called sleep structure or sleep stage distribution.
Total sleep time tells you how much. Architecture tells you what kind. A typical adult night runs through four to six cycles, each averaging around 90 minutes, with the strong caveat that the 90 is an average and not a rule: cycle length varies between people and stretches or shortens across a single night.
The classic shape is lopsided on purpose. Deep sleep is loaded into the first half, the first cycle usually carrying the longest slow wave stretch. REM sleep does the opposite, starting as a few minutes and growing until the last stretch before morning can run half an hour or more. In between sits N2, the largest single share of the night. Brief awakenings are part of a normal night too, not a fault in it, and most of them are never remembered.
What changes the shape
Age flattens it. Slow wave sleep declines across adulthood and nights become more fragmented, which is normal rather than a problem to solve. A stretch of short nights tends to push the body towards deep sleep first when it finally gets the chance, so recovery sleep is not simply more of the same. Alcohol is the classic distorter: it often shortens the time it takes to fall asleep while suppressing REM early and leaving the second half of the night broken. Fever, a new baby, an unfamiliar room and a late shift all leave their own signatures.
This is why a single headline number is a poor summary of a night. Two nights of seven hours can look nothing alike once you see where the stages fell and how many times sleep was interrupted, which is what sleep efficiency and the stage chart are for.
The chart itself is a hypnogram, and Tock draws one in the night detail alongside its stage estimates. On a phone only night those stages are inferred from movement and sound rather than measured, so read the shape rather than the decimals. Measured stages need Apple Health or a wearable. For the order of it all in plain language, sleep cycles explained is the longer read.