A microsleep (MS) is a sudden temporary episode of sleep or drowsiness which may last for a fraction of a second or up to 30 seconds where an individual fails to respond to some arbitrary sensory input and becomes unconscious. MSs occur when an individual loses and regains awareness after a brief lapse in consciousness, often without warning, or when there are sudden shifts between states of wakefulness and sleep. In behavioural terms, MSs may manifest as droopy eyes, slow eyelid-closure, and head nodding. In electrical terms, microsleeps are often classified as a shift in electroencephalography (EEG) during which 4–7 Hz (theta wave) activity replaces the waking 8–13 Hz (alpha wave) background rhythm.