Energy

Sims' energy slowly decreases while they are awake. If a Sim runs out of Energy, he/she will collapse from exhaustion, and fall asleep on the floor. Sending a Sim to work or school with low energy will harm their performance. While they won't collapse from exhaustion away from home, they can do so as soon as they return – and then they'll just be an unconscious Sim littering the pavement. If their energy drops too low while eating, they will fall asleep on their plate.

Sims can replenish their energy by sleeping. Sims will automatically sleep until 6 am or until the Energy bar is back at 100%. In The Sims 3, the player can drag the action handle to specify a time for the Sim to wake up.

If a Sim needs a quick boost of energy, they can drink coffee or espresso (although children can't do that). Sims can get coffee from a coffee machine like the Extra Pep Coffeemaker. Sims can buy an espresso from the espresso bar while at community lots, but really, the best thing to do is simply have your Sims go home when they get too exhausted, as coffee and espresso decreases bladder, and may have other side effects.

Sims also gain energy from sitting down. Sims can also take naps on lounge chairs, recliners, and sofas, but napping doesn't replenish energy as well as sleeping in a bed, and active Sims may wake up before they're fully refreshed.

Sims will not die from exhaustion, though if they become extremely tired while in a pool, they can drown.

Equivalents for other life states
For Servos, the equivalent of Energy is Power. Servos can replenish this by sleeping or napping, the same as Sims can. If it's daylight, they can use the "Recharge" self-interaction to go outside and stand in the sun until their Power is replenished. If they are in gold or platinum aspiration, they can also restore their Power by using The Eclectic and Enigmatic Energizer or drinking the Elixir of Life.

For SimBots, the equivalent of Energy is Charge. SimBots recharge while sleeping, but can also recharge if they get electrocuted.