Microsleep is a very short sleep that occurs when people are awake. It can last from under a second to about 30 seconds. It may happen as a result of sleep deprivation, fatigue, sleep apnea, narcolepsy, or hypersomnia. Microsleeps can occur at any time. Most of the time, they do so without warning. When people have a microsleep during conversation, they may lose track of the subject that was talked about. 