I actually took my daughter to her pediatrician about sleeping too much when she was around 18 months old, because she had been doing it since she was a baby. In any given 24 hour period, she would only be awake for about 4 hours. I thought surely there had to be something wrong with her.
He told me that she's just on the very high end of a normal sleep schedule, nothing to worry about. All kids are different, she would grow out of it.
She did grow out of it, and I remember thinking at the time that I missed her, because she was always asleep! She just needed it, and when she was over it, she was over it. Now she has a pretty normal sleep schedule.
He may be growing through a growth spurt too. I'm sure everything is fine.
Hope that helps!