My 6 month old goes through the night for about 11 hours, and used to sleep for about 5 hours a day starting with 2 hour nap in the morning, 2 hour nap at
lunchtime and then an hours sleep late afternoon. Recently he is getting up after just 40 mins of sleep on his first nap of the day and I cannot get him to go back off. He is yawning his head off when I bring him downstairs so indicates to me that he is still tired. His next nap of the day is likely to be about 90 mins and then about 40-60 mins for final nap of the day. I am so worried that he's not getting enough sleep - is this something I should be concerned about?
You cannot control whether he goes to sleep or even how long he sleeps for. This is under only his control and so it becomes pointless to worry about something you cannot change.
Sleep is a dynamic and ever changing state. What he’s doing now is not a snapshot of how his sleep will always be. It will change with his age and milestones and sometimes he’ll want more and other times less, you’ll find that when he’s going through growth spurts he’ll want more sleep to fuel the energy that growth requires.