My ds is similar and will have smaller cat naps during the day but sleep really well at night. He too was getting really crabby by about 5 o'clock as he hadn't slept well during the day. It used to stress me out so much as I tried to get him to sleep longer. In the end I just went along with what he wanted. This meant that he would sleep for half an hour or what ever and then I would put him back down when he started showing tired signs so he would have 8 or so short sleeps during the day and then long sleeps at night.

It does drive you a bit batty putting them to sleep so often but it worked well for us. In terms of development I dont think it is an issue as alot of babies have this kind of sleep habit. A friend told me that her son was like this until he started crawling and would tire himself out with all the activity. At this stage he started sleeping more. My son has recently started sleeping for an hour to an hour and a half but he did this on his own, just started one day out of the blue.

The good thing about short sleeps is if you want to go out somewhere but bubs needs a sleep you know that they will be up soon and you will be able to go!