Which EEG pattern is absent during drowsiness?

During drowsiness, alpha waves are characteristically present; they are typically associated with relaxed, alert states when the eyes are closed. Theta waves can also appear in drowsy states, especially as one transitions into sleep. Delta waves are often associated with deeper sleep stages, making them more prominent in non-REM sleep rather than in a drowsy state.

K complexes are specific to the sleep cycle and are seen primarily during stage 2 of non-REM sleep. They represent sudden bursts of brain activity and are often preceded by a decrease in alpha activity. As such, in a state of drowsiness, K complexes are not expected to be present. Their absence during drowsiness reflects the brain's transition from wakefulness to sleep and indicates that the individual has not yet entered into the more organized rhythmic activities associated with deeper sleep stages.