I do the same thing Silver Shadow does. I force myself out of bed. Some days it just doesn't happen though. Like today, lol. So tomorrow before I head out for a day at the beach, I have to go in and do some heavy squats. I'll be going in tired too, because I'm at work now and I'm not done until 12. By the time I eat a few more meals, drink a shake, relax it will be 3. And I've been promising my wife a day out, so off to the squat rack at 9 or 10 for me! But with low sleep and only one meal in me, there better not be someone curling in the rack! It happened earlier in the week and the bastard was there for 30minutes.
You know, I never thought making time for the gym would be a problem. When I was younger I never understood how some people who used to be so active could let themselves fall apart. Then you enter the real world and you quickly see how precious free time is.
As for your question, a few years ago, I would have said take the day off and rest. Now I see that you just have to go in there and do it, even if it's a day you feel tired or low on energy. Drink coffee, eat a quick pick me up meal that has complex carbs, some fruit and quality protein. Because it may suck going in the gym with a tired mind and forcing yourself to hoist up the weights, but you always feel better afterwards.