Non 24-hour cycle
Do you mean you sleep
fine for a while, but then you cannot fall asleep at the usual time anymore? You may have a non-24 hour cycle.
Most people have a 25 hour cycle, and readjust by an hour daily. Someone with a longer cycle has a harder time adjusting daily, and eventually cannot adjust - cannot fall asleep on time or wake up on time. Eventually the cycle comes back around with normal sleep.