Why is my baby so energetic?

My son is 5 months old now. He sleeps between 8pm and 9pm everday, and wakes up at about 7-7:30am. During the day, he hardly sleeps. He will seem to be sleepy and tired three times during the day, but each nap is too short, varies between 5 and 30 mins. Why is he like that? I wish he could have a longer nap so I can have things done.I\'m so tired...