I think some of this is due to genetics, Elvis's mum died at 46, Elvis was 42.
Although not in the best health, Elvis most certainly wouldn't have died when he did had he not taken various packs of pills supplied by his doctor that day to help him sleep. He also took quite a few Codeine pills supplied by the dentist (who he visited at midnight). There was 10 different narcotics in his system and 4 of those shouldn't have been taken together - and never with opiates. He was due to go on tour the day he died - the first concert after the book written by his ex body guards was released. It may have been the stress of facing the crowds for the first time since it's release, that prevented him from sleeping - hence .....