Fitting in time to work out

Any moms who've returned to work full time have tips for fitting in exercise. I used to work out everyday but since going back to work it's so hard. 3 days this week I got up at 5am and went before work but have been exhausted the rest of the day. I know I could always go after work most days but I feel guilty and want to spend time with my son! I'm open to any suggestions!