When did your LO drop to one nap, and how did you know they were ready?

DS is usually a great sleeper - he goes down without a fuss for naps and at night, he puts himself to sleep, takes long naps and STTN 10-11 hours. I thought we were miles away from dropping a nap, but now I'm not so sure!

Yesterday he slept 1 hour in the morning as normal, but then he completely refused to take his afternoon nap (usually 2 hours). I tried for 2 hours to help him to sleep! I've never given up on a nap before since he's been in a routine. I chalked it up to a bad day and just put him to bed early, but now he has just done the same thing with his nap this morning. He slept fine overnight so I don't think it's teeth or anything bothering him.

If it's time to drop the nap then I'm sure we can make it work, but I don't want to do it too soon! He's only 10.5 months, but he did drop to 2 naps just shy of 6 months.