Lemme guess... the fan runs only during lights on.
At lights off, the fan shuts down. But the lights are still HOT from having been on, and the room will actually spike up to its max temp in the half hour or so after the lights go off...
If your thermometer has a probe that allows you to look at it from outside, take a peek at what the temps do at the end of 'day'. You may be surprised.

To fix, put the exhaust on a different timer that comes on at lights on but shuts off an hour later.
stinkyattic Reviewed by stinkyattic on . Why do temps go up at night? I've searched and didn't find an answer. I have a Homebox S (31.5" x 31.5" x 63" - 6.89 sq. ft.). We have central A/C set on 76 F 24/7. I'm running 16 26 watt CFL's and 4 14 watters. The ocsillating fan runs in the box 24/7 as do the lights. The exhaust is handled by a 500CFM fan pulling through a DIY carbon filter. Our plants are still vegging so everything is on 24/0. During the day, the ambient temp inside the box runs from 81 to 84 degrees. At night, the inside temp will get up Rating: 5