John Gill Commentary Mark 14:40

"And again he came, and found them sleeping, for their eyes were very heavy; and they knew not what to answer him." — Mark 14:40 (ASV)

And when he returned, he found them asleep again
Notwithstanding the expostulation he had used with them, the exhortation he had given them, and the danger he had suggested to them:

for their eyes were heavy :
with sleep and sorrow:

neither knew they what to answer him ;
partly through confusion and shame, not knowing how to excuse themselves; and partly, through their being stupefied with sleep and grief.