They need to stay warm, depending on their age. If it warm enough in the brooder they don't need the light at all.
Heat lamps can be a fire hazard if not properly used, but the chicks can die if they get too cold. There are other ways of heating the brooder. Do a search in the forums for "mama heating pad" to see an alternative.
A fire is just as likely to occur during the day as it is during the night. If your set up is SAFE, it doesn't matter whether it's day time or night time. Your chicks need the APPROPRIATE amount of heat 24/7. In the summer heat, it's much easier to over heat them. IMO, more chicks die from over heating than being kept too cold.