My Fitbit Charge records total hours asleep then breaks that down into periods of deep/light/REM. I noticed it had stopped recording sleep a few weeks ago and assumed it had developed a fault. However, after following instructions that it required a software update the sleep facility has been restored.
It might be worthwhile checking if yours requires an update Sparklefizz in case there’s a glitch waiting to be cleared.
I believe it only records continuous sleep so if you wake up for any reason sometimes without realising or remembering that period it only records that.