Why are you not suppose to take diet pills before you sleep?

I take Xenadrine and Dexatrim daily and i pop 2 of my Dexatrim with Xenadrine mixed in water before i go to bed and ive had no side affects. So i was wondering why they tell you not to take it within 5 hours of sleeping on the bottle? And also, can you build immunity to diet pills? Do you stop losing weight from them after awhile?