en_tn_lite_do_not_use/mrk/14/40.md

found them sleeping

The word "them" refers to Peter, James, and John.

for their eyes were heavy

Here the author speaks of a sleepy person having a hard time keeping his eyes open as having "heavy eyes." Alternate translation: "for they were so sleepy they were having a hard time keeping their eyes open" (See: rc://en/ta/man/jit/figs-metaphor)