REM sleep is a state in which powerful inhibitory and excitatory mechanisms coexist. Numerous short-lived activities, known as phasic events, occur against a backdrop of sustained muscle atonia and vagal tone during REM sleep. Pioneering research in this area dates back to 1953, with renewed interest periodically emerging in the context of diseases such as depression, erectile dysfunction, sleep-related breathing disorders, and REM sleep behavior disorder. This review in two parts provides a comprehensive list of these phasic activities in the human body. The first part of the review describes rapid eye movements and muscle twitches in the striated muscles of the face, pharynx, larynx, neck and torso, and the limbs. It describes their putative functions. These activities can result in brief movements such as tympanic movements (middle ear muscle activity) and facial expressions. Phasic activities such as middle ear muscle activity, swallowing, smiling and head jerks occur more frequently during REM sleep than NREM sleep; however, rhythmic masticatory muscle activity occurs less frequently during REM sleep than NREM sleep. While some phasic activities occur alongside rapid eye movements, this is not always the case. This calls into question the definition of phasic and tonic REM sleep as distinct sub-phases.