Erratic sleep... something I am very familiar with.
I take a supplement of Melatonin, and it helps a lot. Most of the time, it is discomfort, ill health, pain, etc that disturbs my sleep, at other times, the melatonin helps me.
I suffered some kind of sleep disorder for several years, coupled with nightmares that are referred to as hallucinations in sleep. (I dont remember the exact word for it).
What is important to note is that - we need sound sleep (deep sleep) at night to function well during the day.
Lying in bed sleeping even for 16 hours at a stretch, does not always mean the sleep is 'sound'. As a result, even after that I used to want to sleep all the time. If i was awake i would be restless and not know why.
Little kids usually cannot express such problems well enough.