Charles Ellicott Commentary John 11:12

"The disciples therefore said unto him, Lord, if he is fallen asleep, he will recover." — John 11:12 (ASV)

Then said his disciples.—Better, Therefore... Their remark immediately arises out of what our Lord has said. They are glad to grasp at any reason for not going to Judea.

If he sleep, he shall do well.—More exactly, If he has fallen asleep, he will be saved. There could be, therefore, no reason for His going, as the disease had passed the crisis. Sleep is given by the Rabbis as one of six favourable symptoms, and that it is so is a commonplace in authors of all periods. From the apparent suddenness of the attack, and rapidity of the progress of the disease, it would seem to have been the “great fever” which was common in Palestine (Compare John 4:52, and especially Note on Luke 4:38), and in which sleep would be the sign that the fever had ceased.