I haven't been sleeping that well and am tired in the mornings, so like you I feel like I have no energy to work out. I go anyway and do a long slow first exercise- right now that means 30 minutes on the stationary bike. I bring a magazine and at first just start pedaling leisurely at level 6 (of 10). By about 15 minutes in my heart rate is up and the adrenaline is pumping. I put down my magazine and start pedaling faster. By the end I fee like I have tons of energy and hop on the treadmill for a run, then onto weights.
This method has worked really well for me- just starting out slow and leisurely and tricking my body into waking up. It does, and then I go for it
As someone else said, just GET to the gym and start moving and you'll be set- finding the motivation to actually walk through the doors is half the battle!