drowsy
寎 centers on a state of drowsiness or sleepiness, often associated with being half-asleep or in a dreamy, lethargic state.
寎 is a rare character. It likely combines 宀 (roof) with 爿 (bed) and 丙, suggesting a person lying down under a roof in a drowsy state. The exact historical development is uncertain.
Under a roof (宀), a person lies on a bed (爿) with a heavy head (丙), feeling too drowsy to get up.
For ヘイ, imagine a drowsy person saying 'hey' (ヘイ) in a sleepy, drawn-out voice while lying in bed.