Energy

Sims energy slowly decreases when awake. If your Sim runs out of Energy, he/she will collapse from exhaustion, and falling asleep on the floor. Sending a Sim to work or school with low energy is not good, and although 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, but really, the best thing to do is simply go home when your Sims 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 espresso (though the latter decreases bladder, and may have other side effects). Their energy slowly decreases when awake but decreases a lot when working out. Also it decreases more than little when at work. Sims can also take a nap on lounge chairs and sofas but it increases very slowly.

Motive Depression
There is no known death if a sim is low on energy, but they will drown when they are in a pool. However, they will pass out if their energy is completely red. If they are eating food and their energy drops low, they will fall asleep on their plate.