Needs

When Sim needs are high, a Sim will be Happy, unless something happens to cause another emotional state. When Sim needs are low, a Sim will be Uncomfortable. Filled needs bars are green, low needs bars are red. To prevent uncomfortable Sims, fill needs when the bar is yellow.

The most important Sim needs are: Hunger and Energy, followed by Bladder.

Hunger
If a Sim's hunger needs are not met and the hunger bar becomes empty, the Sim will starve to death. Before this happens however, the Sim will become Hungry, then Ravenous. Eating a cooked meal will generally fill up the hunger bar. Eating a quick meal will partially fill up the hunger bar. The hunger bar will deplete with time.

Energy
If a Sim's energy needs are not met and the energy bar becomes empty, the Sim will collapse on the floor from exhaustion and fall into a sleep that the Sim cannot be awakened from for a certain period of time. Before this happens, the Energy bar will turn yellow, then when it first turns red, the Sim will become tired, then exhausted. If a Sim in a pool collapses from exhaustion, it dies from drowning. To refill the Energy bar, Sims need to Nap or Sleep, either on a sofa, or on a bed.

Bladder
If a Sim's bladder needs are not met and the bladder bar becomes empty, this is the equivalent of a Sim having a full bladder, and the Sim will wet themself, leaving a pee puddle on the floor, and reducing the Sim's hygiene need bar to 0 hygiene (empty hygiene needs bar). Using a toilet, a bush, or leaving a puddle on the floor refills the bladder needs bar.

Hygiene
If a Sim's hygiene needs are neglected, the Sim will become Grungy. If hygiene needs drop lower than Grungy, the Sim will have problems with Social interactions. To restore hygiene, a Sim must take a bath or shower. Certain types of showers can result in certain emotional states. A thoughtful shower, for instance, will result in an Inspired Sim. A brisk shower will result in an Energized Sim, and a Steamy Shower will result in a Flirty Sim. There are also some bath options that will result in emotional states.

Social
If a Sim's social needs are neglected, the Sim will become Lonely, then Desolate. Lonely or Desolate Sims are Sad sims. Social interactions are necessary to join and maintain status in Clubs, which were introduced in Sims 4 Get Together.

Fun
If a Sim's fun needs are not met, the Sim will become Tense. Tense Sims are usually unable to use skill objects. Work can reduce fun need (especially if the option to Work Hard is selected), and the fun need bar goes down with time.