Hi Y. - We had the same thing. Our baby slept great - then at 5 months it was hell. Waking up three or four times a night. I thought I had died and gone to hell. After three months of losing my mind, I got the books - Healthy Sleep Habits, Happy Child and The Happiest Baby on the Block. Saved my life. I shouldve gotten them sooner. The problem is that around 4 to 5 months their brains change and they start to become aware of sound and things around them, so they dont sleep as well. Get those two books and they will sleep through the night. The first mistake you are making is the same mistake I made, putting the baby to bed too late. At 8.30pm it is WAY too late for them. He needs to be in bed by 6.30pm or 7pm latest. I made this one change and in a week he was sleeping 11 hours - only waking up one time for a feeding! It was amazing. Dont let him cry himself to sleep. It doesnt work. Dont listen to anyone who tells you that. It DOES NOT WORK. Also, you need to swaddle and shhhh. I also have an air filter in his room that cuts outside noise and is soothing. This is recommended in the book. Also, from 4 months to 7 or 8 months, do not let them go more than 3 hours without a nap. Keep putting him down during the day. This is SOOO important, or else they are sleep deprived and are overtired during the night and wont sleep. Read the books. They are life savers. Here is my schedule that I started at 6 months to help you. It has changed as he has gotten older (he is 9 mos) - everything is within an hour time frame. Obviously you are not feeding solids yet, but you will at 6 months. this will also help sleeping through the night. If the baby wakes more than once during the night, he is not hungry and you need to just rock him back to sleep. All naps during the day should be at least an hour. If it is less than that, they are not sleeping enough.
Baby wakes - btw 6am and 7am, feed bottle or breast as soon as they wake up
Breakfast - depending on wake up time btw 7.30am and 8.30am, Playtime - right before nap
Morning Nap - Feed bottle or breast - btw 8.30am and 10am
Playtime - Usually I would walk outside, this is very good for brain development and getting them tired
Noon Nap - btw 12pm and 1pm
Lunch - 1pm (bottle and food depending on age)
Playtime - If I didnt walk outside in the morning I try to do it now. I also use tapes and read to him
Afternoon Nap - Depending on age btween 2pm and 4pm
Dinner - btw 5pm and 6pm
Bath - 6pm to 6.30pm
Bedtime - Bottle or breast and in bed by 7pm latest
As he gets older, the baby will get rid of the noon nap and the afternoon nap will get a little earlier like 1.30 or 2pm. But it is very important to watch for signs of sleepiness before they get to tired. Some children need four naps a day - others three. Some children get rid of their morning nap at 9 mos. the others the afternoon. All children are different. You just need to read the books and watch the signs so your child doesnt get overtired!! Hope this helps!