Waking up briefly at the end of a sleep cycle approximately every 90 minutes to make a postural movement is normal.

“part of that is because they're going through very reliably timed sleep cycles and every time we finish a REM sleep period we wake up but it's normally very brief and the reason is because we've been in paralysis and the body needs to move so we wake up we make a postural shift we move in our bed just slightly and then we go back to sleep”

“and that being a very common issue is as far as I understand reflective of the fact that early in the night our sleep is dominated by slow wave deep sleep with less rapid eye movement sleep and then somewhere right about that transition time it's not necessarily 2 or 3 a.m. per se but given that you asleep for about 3 4 hours after about 3 4 hours of sleep the proportion of our sleep that is rapid eye movement sleep relative to deep slow wave sleep shifts dramatically”