Baby always tired?

this might be an obvious answer of babies always get tired but my 5 month baby seems constantly tired. Her bedtime is usually approx 5pm and she sleeps till around 11pm then feeds then sleeps till approx 7am. She wakes sometimes a couple of times just for some company or her dummy going in but genrally has a good night sleep, she wakes in the morning and is fab for about an hour and a half then she is so tired she gets upset. then goes asleep, well she really fights it rubbing her eyes and dozing off but then wakes again, sometimes she goes asleep in her cot but sometimes we have to put her in pram which she then falls asleep in again, then its awake again another hour then needs to sleep, the sleeps are anywhere between 20mins to45mins she will not take any more than that. All the other babies in the baby cles seem full of energy. My baby does play a lot when she is awake, jungle gym etc!