sleep
寐 centers on the state of sleep itself, without additional nuance of lying down or resting.
寐 combines 宀 (roof) with 爿 (bed) and 未, suggesting sleeping indoors on a bed. The exact role of 未 is uncertain; it may be phonetic or a semantic extension.
Under a roof 宀, on a bed 爿, with the 未 component looking like a person curled up asleep: 寐 means sleep.
For ビ, imagine a bee buzzing softly near the sleeper under the roof: bee -> ビ, and the gentle buzz lulls them deeper into sleep.