Sleep glossary
Total sleep time (TST)
What total sleep time counts, how it differs from the hours you booked in bed, and the duration guidance for adults that is worth paying attention to.
In short
Total sleep time, or TST, is how much you actually slept in a night, once the time you spent awake in bed is taken out. It is time in bed minus sleep latency and minus wake after sleep onset.
Total sleep time (TST) is also called TST, hours slept or actual sleep time.
The distance between total sleep time and the hours you meant to sleep is where most disappointment with sleep trackers lives. You booked eight; the app says seven. Nothing is broken. Eight hours of time in bed minus 20 minutes falling asleep, minus 25 minutes of wake after sleep onset, minus a quarter of an hour staring at the ceiling before you got up leaves 420 minutes of actual sleep, and that is an ordinary night at roughly 88% sleep efficiency.
How much total sleep time you need
The AASM and the Sleep Research Society put adult guidance at 7 or more hours a night for adults aged 18 to 60. That is a population floor rather than a personal target, and individual need genuinely varies around it. The more useful test is behavioural: can you wake at a stable time without needing an alarm to drag you out, and get through the afternoon without fighting to stay awake. Chronic shortfalls accumulate as sleep debt, and that debt is repaid over several nights rather than in one long weekend lie-in.
Averages carry more information than any single night here. One short night is noise. Two weeks averaging six hours when you need seven and a half is a pattern, and it usually shows up in mood and reaction time well before it registers as feeling tired, because our sense of our own sleepiness is a poor instrument for measuring a deficit.
On a phone, total sleep time is an estimate. The app infers when you fell asleep and when you were briefly awake from movement and sound, and quiet wakefulness gets miscounted as sleep more often than the reverse. Tock shows the estimate for each night with its parts broken out, so a short night points somewhere: late to bed, or an hour lost in the middle. If you are working backwards from a fixed wake time towards a bedtime that actually delivers the hours, the best time to wake up does the arithmetic properly.