I know that most of my childhood memories are gone. Usually attributed to old age. Except this can happen as many as a dozen times a day, or more. It doesn't happen every day. So the unpredictability is also a factor.
Sleep pattern disturbances.
Sometimes you want to sleep for days. Sometimes you can't sleep for days. Sometimes 15-20 minutes of sleep, and you're good to go for another 36 hours. Another thing that can happen is when you are just about at the point of sleep and any slight noise will wake you up. When this occurs, you feel as though you've had two days rest, and are ready to go for another 10-12 hours.