The Energy Motive slowly decreases when awake. It's regarded as one of the more important Motives, alongside Bladder and Hunger. If your Sim is unlucky enough to out of Energy, they will collapse from exhaustion, and will sleep on the floor, drastically lowering their Comfort.
 How To Resolve
Quick fixes for this involve drinking coffee, and napping on the couch; however these do not work as well as the most obvious- sending your Sim to bed. Sims will automatically sleep until 6AM or until their Energy bar is fully green.
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