Which function expresses the frequency of eye movements per unit time during the sleep stage REM?

The function that expresses the frequency of eye movements per unit time during the sleep stage REM is referred to as REM Density. This term specifically relates to the number of rapid eye movements (REMs) that occur within a defined time frame, typically measured in epochs during a polysomnographic study. High REM density indicates a greater frequency of eye movements, which is a characteristic feature of REM sleep.

In contrast, other terms like REM Frequency would suggest the overall count of eye movements without specifying the time aspect, while REM Duration refers to the length of time spent in the REM stage and REM Amplitude indicates the intensity or strength of the eye movements, not their frequency. Thus, REM Density is the most precise term for describing how frequently eye movements occur in REM sleep.