[Pebbles82]

I wouldn't put any kind of nasty tasting thing on the bars weaving cardboard can help. But it would be good to know why he is doing it. Has it just started or has he always done it?

As Fluffagrams said it can be something he's not happy with in the cage - eg the wheel not working properly. There are other things too - if there isn't much substrate, or if there isn't anywhere dark to hide away (eg a large enough house or under a shelf).

Another thing can be cleaning out too often, which stresses them. As the others say, a photo of your cage set up might help give some suggestions to help him settle.

Or - he could just be a chewer and have developed this habit. Some changes might distract him if they're the right changes.

Or just a very active hamster who wants more space at night! But I think if they're happy with their set up and have a good wheel, they don't really want to get out of it at night. What size wheel do you have? If it's too small he may not be using it as a wheel that's too small can cause back pain - and then he won't be getting enough exercise which could lead to pent up energy and bar chewing.