Asleep is an emotion in The Sims 4. Sims will always feel this emotion while they are sleeping, since the "Sleeping Soundly" moodlet shows 10,000+ points towards the asleep emotion, overriding any other emotions except for the possessed emotion, which is caused by a moodlet that has the strength of 7,548,431+ points.
For servos asleep is renamed to "charging". When a servo falls asleep, it gains a moodlet that adds +1,337 towards the charging emotion, with the same function as the regular asleep moodlet.[TS4:DU]