LodestarREM is the stage where most vivid dreaming happens. Most adults spend about a fifth to a quarter of the night in it, and the easiest way to lose it is to cut your sleep short.
What REM sleep is
REM stands for rapid eye movement. Your brain becomes very active, close to waking levels in some areas, while most of your muscles go slack so you don't act out your dreams. Breathing and heart rate become less regular than in deep sleep. If your Fitbit shows heart rate wobbling around in the early morning, REM is often why.
REM seems to matter for emotional processing and for some kinds of learning and memory. The research here is still developing, and it is fair to say scientists agree it matters more than they agree on exactly why.
How much is normal
In healthy adults, REM usually makes up around 20 to 25 percent of total sleep. On seven and a half hours, that is roughly 90 minutes to just under two hours. A large analysis of sleep studies found REM share stays fairly stable through adulthood, falling only slowly with age, unlike deep sleep, which drops much more (Ohayon and colleagues, Sleep, 2004).
The timing is the part most people don't know. Each night runs in cycles of about 90 minutes. Early cycles are heavy on deep sleep with short REM periods. Later cycles are the reverse, with REM periods getting longer toward morning. The last two hours of a full night hold a large share of your REM.
So if you normally sleep eight hours and set an alarm for six, you don't lose a quarter of everything. You lose a much bigger share of REM, because that is what the end of the night was mostly made of.
What cuts REM short
- Short nights, especially early alarms, as above.
- Alcohol. It tends to suppress REM in the first half of the night. Your body may try to make it up later, often with lighter, broken sleep.
- Some medications, including several antidepressants, reduce REM. Don't change any medication because of a sleep tracker. If it concerns you, ask your doctor.
- Irregular timing. REM is tied to your body clock as well as to how long you have slept. Shifting bedtime and wake time around makes the pattern less predictable. See why a consistent sleep schedule matters.
After a few nights of short or disrupted sleep, you may see REM rebound, with more of it than usual once you sleep longer. That is your body catching up, and it's normal.
Reading your Fitbit's REM number
Fitbit estimates REM from heart rate patterns and movement. It is an estimate, and stage-by-stage accuracy on any wrist device is lower than its accuracy at telling sleep from wake. Treat it as a trend line.
- Compare with your own average over a couple of weeks.
- If REM is low, check total sleep and wake time first. That is the usual answer.
- Look for patterns with alcohol and late nights. They show up clearly for most people.
Low REM on a tracker is not a diagnosis of anything. If you feel exhausted most days despite enough time in bed, or you physically act out dreams, mention it to a doctor.
How Lodestar helps
Lodestar is an iPhone app that reads your Fitbit sleep stages through Google Health. Its Sleep score weighs how long you slept against your own sleep need, then REM, deep sleep and efficiency against your usual range. "What Moved This Score" shows exactly how many points REM added or took away, so a short REM night after an early alarm makes sense at a glance. Everything is worked out on your phone. For the other half of the picture, see how much deep sleep you need.