In the wild bps spend a lot of time in old rodent/termite burrows which I believe tend to be pretty consistent temperature wise both the day and night
I'd just keep it on 24/7, since most bps prefer eating at night it's also probably good for their digestion to have the hot spot still available for them then. Just a thought(assuming you're using uth or Che... I would not use a light emitting bulb at night.)
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