This is my first summer to care for a garter. I am wondering how the season change affects his heat lamp needs. I live in Texas and it gets HOT.

Sugar the snake has both a lamp with a special herp 40 watt blue bulb atop his screen top and a heat pad under the tank. Over the winter and spring his lamp has been set for a 16 hour light cycle. Now that summer is approaching and his ambient room temperature is around 80 - but varies - old house without central AC - shall I lessen his lamp time?
(He also will stay with a friend when I am out of town in a few weeks and they do not have much AC so guessing could be 90ish ambient room temperature.)
He has been hiding out more under the bark in the day, but could be many factors - he just went into shed, added nightcrawlers to his diet last month, or maybe the increase in heat?
thanks for the advice