Many people awaken once or twice during the sleep period. They usually can submurge again within 15 minutes to a half hour. Sleep is not usually down in depth for the entire sleep period but has waves of depth and emergence in cycles throughout night. All within normal limits.
Problems arise with extrememly shortened or absent sleep.
I recall having studied it in a college class...psychology?
IIRC, tbl is caused when there is lack of Stage IV or REM sleep, or if a person awakens during REM they are more apt to feel tired or edgy...all this has something to do with folks who are able to take "power naps" but I can't intelligently say how any more since I took that class when I was about 18!