system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we require less sleep as we get older?
gpt
Our body has a thing called a "Circadian Rhythm" which is a posh word for "I get tired at nighttime". It runs off various things like light and social cues (why eating at the right time can help you deal with jetlag). For most of our lives, this runs at 24hrs so over the course of a day, we get tired for about 8-9 hrs at night so we sleep but are less sleepy during the day (due to melotonin levels in the brain). As you get older, this extends, up to 48hrs in some cases. As such, your body only wants the 8-9hr sleep over two days as opposed to one. 

The other, slightly less exciting explanation with fewer 'big words' is activity levels. Most pensioners work less and do less as opposed to say 25 year olds who work full time jobs and have a hectic social life. Therefore, when we are younger we do more stuff so need more sleep. As we get older, because our bodies can't do as much stuff anymore (illness, age, arthritus etc) we expend less energy so need less sleep. 

Overall, the predominant factor is the circadian rhythm thing but the activity levels do play a role. If you are interested, look up "Siffre's cave study". Hope this helps!!