My DS stopped sleeping very well during the day at around 4-5 months. He couldn't resettle after a sleep cycle, so I was only getting 3 40-minute naps out of him. It was doing my head in too!

What worked for me was going back to wrapping him (angel wrap) for his day sleeps. We restarted that at about 6 months and he went to 2 hr naps! To start with I had to do controlled comforting (patting him to sleep while he lay in his cot, on his side) but pretty soon we didn't even need that.

Anyway, all babies are different, as I'm sure you know. I hope you find out what works for your DS!