I would like to add to the excellent replies to this post that the temp should be measured right over the under the tank heater using a digital thermometer with a probe or a temp gun

You should not be measuring the air temps to read 88 to 90 degrees as overall tank air temps that high will be too hot for your gecko

Also-if you have an albino/red eyed Leo-which is sensitive to bright white light and have such a light on the tank-get rid of it

I hope that your geckos is OK and that this has helped