Why won't my 5 month old stay asleep early at night?

My five month old son falls asleep very easily by himself in the crib during the day. I'll put him in the crib still awake and he'll fall asleep easily within just a few minutes. At night, I'll put him to bed around 8pm, sometimes as early as 7:30 but he wakes up around 9:30, sometimes even as late as... show more My five month old son falls asleep very easily by himself in the crib during the day. I'll put him in the crib still awake and he'll fall asleep easily within just a few minutes. At night, I'll put him to bed around 8pm, sometimes as early as 7:30 but he wakes up around 9:30, sometimes even as late as 10:00 and then I can't get him to fall back asleep. Two nights ago he woke up at about 9:20 and didn't fall back to sleep until almost midnight! I always try to get him to fall asleep on his own first but even with me rocking him he won't fall back to sleep. I might think that he was acting like it was a nap but he acts like he's still tired. This wouldn't be such a big issue if it were just me and my husband (who has to get up at 5:30) but we have a two year old daughter and worry about him waking her up! Any tips would be greatly appreciated.

He's breastfed but we give him a bottle of formula before bed to help him sleep longer which he takes with little issue. During the day he takes about an hour nap every two hours otherwise he gets crabby. He wakes up for the day sometime between 7:30 and 8, later if he's up later (like when I'm up with him until midnight).

Please help!
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