I agree with LG too, but then I'm a huge proponent of Mama Heating Pad. No lights on them 24/7, far less risk of fire, chicks don't overheat because they are too warm, and it's so much more broody-hen-like. At night even day old chicks go to sleep when the sun goes down and wake up at sunrise, so I don't have to get them "used" to periods of night and day. If you've ever heard a flock of young chickens fuss the first time they are put outside and there is no constant light, you know how beneficial that is to them and to the worried owner! Self regulation is the biggest benefit.
I raise chicks outdoors in a wire pen in the run, even during our Wyoming springtime temps in the twenties, sometimes dropping into the teens at night, and I use nothing but a heating pad "cave". They are totally off heat by 3 weeks, even out there, and fully integrated with the flock by 4 weeks, at which time the pen is taken down completely. Even Silkies - generally thought of as "delicate" - thrive.
But that doesn't answer your question for this batch of chicks. Chicks do need warmth, and if you think about it what time of day is it coolest either outside or in your house? Yep, nighttime. So if you are using a lamp, then nighttime is when they need a little more heat. As has been said, make sure your brooder is big enough so that they have a cool spot to go to and only go under the light to warm up. Wean them off as soon as you can, starting in the daytime.
