Seasonal behavior change?

It's been weeks of warmer temps and he's been in his normal spot and routine and suddenly today he was hanging near the bottom and tonight sitting on the floor of the cage?

He's got a IR light for the winter for about 3mths and there's plenty of crickets in the cage too? Is this perhaps a seasonal change and time to turn off the IR lamp?

It's currently about 69-70 with lights out in the room.
2x a day 90sec misting
4' cage with hibiscus pythos and fake vines. 40-50% humidity

He's about 18mths old too.

By "IR light" do you mean an infra red light at night? Because I would never recommend that. Chameleons benefit from a completely dark night. People here have actually done studies and found that infra red night lights retard growth. That has to translate to: not so good for adults either.

Definitely turn off the night light. It can get down to 55 degrees at night without it being a concern (some will say lower). If your home does get that cold at night in the winter, there are "no light" heat options out there which would be much better.

Screameleons told me to yes use an IR to keep the night time temp up as I'm in New England and it can get chilly during the winter. I keep the door to his room closed at night because I have a curious cat who I don't trust. However the temp has never gone lower then 58 I would say.

Your best bet would definitely be a Ceramic Heat Emitter. You can get them at most pet stores and online. They give off heat and don't give off light. Chameleons can see color, the IR and "Moonlight" lighting are mainly for nocturnal reptiles. The Ceramic Heat Emitter comes in different wattages, such as a bulb. Chameleons benefit from a temperature drop at night. Most people say that if your temp gets as low as 55 degrees you should use one. You're pretty close to that. You might want to get a heat emitter that doesn't get as hot as day temps. Get one that will keep it around the low to mid 70's. That way your Cham will still have a small drop temp during the night, but won't be too cold.

He had been sleeping under the IR light some nights. I'm curious as to why the sudden bottom dwelling? It wasn't crazy warm today either?

Ditch the light. Natalie is absolutely right that he needs a ceramic heat emitter. It's possible that your house is cold enough that additional heat is needed. The warmth felt good so your chameleon parked under the light, but the light disturbed his sleep which is why he is now hanging out at the bottom of the cage.

One thing about chameleons is that they are often reacting to something LONG TERM when we suddenly notice a change. I guess we're a bit slow....