As I wrote about a while back, it is still amazing to me the benefits of turning out the lights earlier at night. Just by pushing the lights out time a half hour earlier, I feel better and more energetic the next day.
What I've come to realize is that the benefits don't come from more sleep, but seemingly from just more time spent in the dark. I tend to sleep the same total amount even when turning out the lights earlier. From this, I would argue that there are two nocturnal evolutionary needs in humans: sleep time and darkness time, which are separate concepts.
Here's some wild speculation on the subject. When I don't get enough darkness time but do get enough sleep, here's what I observe: an occassional and slight deficit in coordination. By this, I mean that I am more likely to say, knock something else over when I'm reaching for something. Or stumble. Something along these lines. Now it's not like I'm fumbling and tripping all day, but I do notice it is more likely to happen when I don't get enough darkness time.
I would say that this points at some sort of hormonal, HPA axis-type stress that is involved with lack of darkness time. Darkness time is a time for the central nervous system to rest, whether you are sleeping or not. Cutting into this CNS recovery time (even given sufficient hours fo sleep) can leave you at a coordination deficit the next day.
This reminds me of a friend from college who had to periodically work the night shift. He said the next day he was always so clumsy. When he tried to fix cereal, he would drop the spoon, spill the milk, etc. At the time, it was obvious to attribute that to lack of sleep. But now I think lack of darkness (given that he would sleep in the afternoon) played a role.
In the evolutionary era, the amount of darkness time was "pre-determined" by the seasons, as and such no one could avoid it. Nowadays with electric lighting and devices, obviously it can be avoided. By the way, I know this has all been said in the book "Lights Out", which I read a decade ago.
After testing, I think it would be cool if some authority-type group added darkness time to health guidelines. For example, they could say that the average adult needs 8 hours of sleep and 9 hours of darkness (or something like that).
Are there any studies out there on darkness time (as opposed to sleep time)? I didn't find any, but hopefully it has already been studied in some form.