Recently retired and now in my early 60s. Currently, I sleep for about 4 - 5 hours then I want to be up. I no longer keep my long-term working schedule (up at 4:30 AM - asleep by 9PM). Just the change in schedule impacted my sleep patterns. Since I no longer need to wake at certain times, I no longer need a solid 7-8 hours. I don't know about melatonin but i never liked the supplements... I felt like I was sleeping while still conscious or something. I never feel rested after taking that gunk.
Regarding the time changes, I always did wake and go to work in the dark. Sunrises make it worth getting up that early.