Leopard gecko HELP ???

ok so ive got my leopard geckos in a new tank but instead of a heat mat ive got a heat lamp and its a 100 watt exo terra heat lamp the temperature is about 85 F so do i to put a heat mat in there aswell if i do on what side of the tank and how long should i leave the lamp on in a day like 12 hours in day or at night etc