Hi K T,
My daughter (who will be 6 months on October 8th) has also been a really good sleeper at night since she was about 8 weeks old. We did have a going to bed issue a month ago. She just wasn't going to sleep at her normal bed time. But I think over this last month we have made great progress in getting her to bed at a decent time. Here is what I have been doing, and it seems to be working for us so far.
One thing I have figured out is that we have to stick to a schedule. If we don't, she will not go to sleep at a reasonable time. By schedule, I mean, we get up at a certain time every day(I work full time, so we have to be out of the house by 7:30 am, which means everyone needs to be dressed and fed before we leave.), and she eats every 2 to 2 1/2 hours. She usually naps 3 or 4 times during the day/early evening. Then at 7:00 every night we start making our way towards the bedroom. Activities become quieter activities and we try to give her cues that its time to start settling down for the night. By 7:30 she is usually rubbing her eyes and showing signs of tiredness. I give her the last bottle for the night at 8:00 and she is usually asleep by 8:30-9:00. She will typically sleep through the night and wake on her own at 7:00 am.
Anyway, the idea is to send cues to your child that its time for bed. I know that at 5 months, they are still making their own schedule, but it has really saved my sanity to have a "family schedule." It takes some adjusting for everyone to get used to it, but in the long run, its been worth it for us. Hopefully we will be able to continue this schedule!
Good luck!
Jenn