For sleep, one needs endless depths of blackness to sink into; daylight is too shallow, it will not cover one.

The line treats sleep as a kind of surrender that requires total enclosure—darkness imagined as “endless depths” that can fully absorb the self. Daylight, by contrast, is “too shallow”: it exposes, keeps one partially awake, and cannot provide the protective cover needed to let consciousness drop away. Beyond the literal preference for darkness, the metaphor suggests a psychological need for privacy and oblivion—rest as a temporary disappearance from demands, scrutiny, and thought. The imagery also hints at the sea (depths, sinking), aligning sleep with immersion and release rather than mere inactivity.