Can 500 mg cephalexin be taken 3 times per day instead of 4 times? i can't see how anyone who sleeps at least 8 hours would be able to take 4th dose.

No: Reducing the frequency of an antibiotic can reduce its effectiveness for your infection. That may actually make the infection worse (more virulent) or even cause it to develop resistance - which will land you in the hospital. It doesn't have to be exactly every 6 hours. If you sleep 9p-5a, take it at 6a, 11a, 4p, 9p. There's a reason the doctor wrote 4x/day - so do it! don't make excuses.

Cephalexin: You can squeeze the four doses into the 16 hours a day that you are awake, or if fewer, then divide that number by 4 and take the medication then.