What defines the first epoch of any stage of sleep?

The first epoch of any stage of sleep is defined by sleep onset. Sleep onset refers to the moment when an individual transitions from waking to sleeping, marking the beginning of the sleep episode. In the context of polysomnography, this is crucial as it indicates when the individual has officially entered a sleep state, whether it be light sleep, deep sleep, or REM sleep.

During a sleep study, the electroencephalogram (EEG) readings provide distinct patterns that denote different stages of sleep. The transition from wakefulness into sleep leads to characteristic changes in the EEG, and identifying the first epoch of sleep is essential for scoring sleep stages accurately. Therefore, understanding sleep onset gives clinicians insight into the patient's sleep architecture and can aid in diagnosing sleep disorders.