Housing upgrade help.

Should I upgrade to a bigger exo terra glass terrarium or just buy a 40 gallon tank? Right now I have an 18×24×18 tank and my little one isn't so little, he's been really restless at night and I don't know if it's because the tank is too small for him now but he will run back and forth between things and won't sleep until two hours later, I was planning on upgrading his tank anyway but I'm not too sure in which direction to go, any suggestion help. Please and thank you.