Energy

Sims energy slowly decreases when awake. If your 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 is not good. While they won't collapse from exhaustion away from home, they can do as soon as they return – and then they'll just be an unconscious Sim littering the pavement.

Sims can replenish their energy by sleeping or drinking coffee. Sims will automatically sleep until 6 am or until the Energy bar is back at 100%.

Energy in The Sims
Sims also gain energy from sitting down. If a Sim needs a quick boost of energy, they can drink coffee or espresso (although children can't do that).

When you go out of the house to places such as Studio Town and Old Town, your Sim's Energy bar will naturally decrease. Your Sim can buy an espresso from the espresso bar, and you can place coffeemakers on community lots, but really, the best thing to do is simply have your Sims go home when they get too exhausted.

Pets

Pets lose energy quite quickly if they are running around, and so need to sleep about twice a day. It is therefore essential that you provide them with some baskets or doghouses for them to sleep in.

Energy in The Sims 2
In The Sims 2 the only real way to replenish your energy is to sleep, or drink coffee or espresso (though the latter decreases bladder, and may have other side effects). Sims' energy slowly decreases when they're awake, but decreases more when they perform physical activities such as working out. A Sim's energy decreases more than a little when he or she is at work or school. 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.

Motive Depression
Sims will pass out if their energy is completely red. If their energy drops too low while eating, they will fall asleep on their plate. Sims will not die from energy depletion alone, but will drown if they run out of energy while in a pool.