lethargic

sluggish or drowsily inactive

Lethargic joins the Lethe forgetfulness image with idleness: the mind and body feel slow, dull, and hard to rouse.

After three nights of poor sleep, he felt too lethargic to read carefully.

Lethargic is deeper than lazy. The older history runs through forgetfulness and inactivity, so the modern feeling is foggy: energy is low, motion is slow, and attention keeps sinking.