No, all the TG lounges close sometime around 0100 or 0130, depending on when the last flight leaves. They reopen again around 0500 or 0515, though some open much later due to recent staff cutbacks.

If you have a Priority Pass card and can get free lounge access, there's a Louis Tavern F lounge (actually not even as good as most C lounges, but they call it 'First class') on Concourse G that I use when I arrive very late at night and leave early in the morning. Technically they will only allow you to use the lounge for 2 hours, but I've always gotten away with staying there for about 4 hours. If you're there for longer than that though, they might notice and kick you out or ask to run your card through again. I just put two chairs together and can get a bit of sleep, then at 0500 head to a TG lounge for a shower and onto my connecting flight.

Louis Tavern also has dayrooms on Concourses A and G, Level 3, but they cost something like 2,000 THB for 4 hours. Not really a good deal, but if used for only 4 hours it's probably the cheapest/best option if you really need some sleep, being the Novotel outside the airport is much more expensive and cheaper options will waste quite a bit of time to get to, not to mention you might have to pay an extra 700 THB airport fee if you exit through passport control.

There is also a free 'transit lounge' at BKK, at the end of concourse B, level 3. It's just a public open area with seating, that anyone can use for free. At night there's a lot of passengers that claim a row of seats and lay down and try to get some sleep. The cheapest way to do it, but certainly not recommended by me. If you get cold easy, you'll likely freeze there, as the airconditioning at night works extremely well. Also might not be so safe for your belongings if you fell asleep and someone decided to swipe your bag.