The Sims Online:
Energy

In essence you have to sleep occasionally, just like in real life! Sleeping is a very quick action - more like a nap. Most homes you wander into have several guest beds set aside for this purpose. Just hop into one (you automatically change into your PJs) and take a nap to get your bar back into the green. If you don't, you'll collapse whereever you happen to be standing.

The Sims Online Tips, Techniques and Walkthroughs