Grandfather wakes up hungry in the middle of the night. Any ideas of what might be the problem?

My grandfather is 87 years old and lives on his own. His wife died 7 years ago which was hard on him emotionally but he has been hanging in there. He's always had a hard time sleeping and whatnot but so far so he's gotten by. Only until recently (for several months now), he's been complaining that he wakes up hungry in the middle of the night. The doctor prescribed him some sleeping medicine to help him sleep deeper and not wake up. This was helpful for a while; he didn't wake up hungry and slept until morning. However, now the waking-up-hungry symptom is kicking back up again and now I am worried if it is a symptom of something else going wrong. The obvious answer is to just have him eat more at night but he isn't hungry and besides, he's been doing the same thing for the last several years without a problem. This whole waking-up-hungry thing only started a few months ago. Does anyone have any ideas of what might be the problem?