Which phenomenon can occur both prior to sleep onset and after arousal?

The phenomenon that can occur both prior to sleep onset and after arousal is hypnagogic hypersynchrony. This term refers to a specific type of EEG pattern that is characterized by bursts of synchronized cortical activity, typically seen during the transition from wakefulness to sleep.

During the hypnagogic state, which occurs as an individual begins to fall asleep, there are often episodes of increased slow wave activity and bursts of theta waves, leading to hypnagogic hypersynchrony. This phenomenon can also be observed during arousals from sleep, indicating the brain's response as it transitions between different states of consciousness.

Understanding hypnagogic hypersynchrony is important because it highlights the dynamic nature of brain activity around sleep onset and after arousal, illustrating how the brain's electrical activity reflects changes in alertness and sleep stages. This knowledge can be particularly valuable for interpreting EEG recordings in clinical settings.