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日 (sun) — the indexing radical. The position is critical: with
氐 above and
日 below, the picture is the sun ducking beneath the horizon — twilight, dusk. From the literal sense came the metaphorical 'dim, faint, dazed, unconscious':
头昏 (dizzy),
昏迷 (in a coma),
黄昏 (twilight).