I suggest that your read the works of Professor Roger Ekirch on preindustrial sleep. His research shows that 8-hour sleep appeared with the industrial revolution. Before that, the general habits of people were to sleep in two periods, getting up in the middle of the night and doing some activity for an hour or two, then returning to bed. Of course, his research has been on pre-industrial, which means agriculturist. I am not sure it would extend to H-G societies. It's been a couple of years since I read his book. You can find an essay by him on the subject online.

Here is an interesting article that includes some research on H-G societies. My facebook bud Jessa Gamble also did an interesting TED speech on this subject here.

When you think about it, bi-phasic (or polyphasic) sleep makes sense from an evolutionary perspective. Think about a tribe in which all members slept through the night. They would be very vulnerable. It would make sense that some members (or all) woke up at various times during the nighy, thereby acting as de facto sentries.

One thing I've done recently is to wear blue-blocker safety glasses from the time the sun goes down until I pop on my right before bedtime. Blue light, specifically of the wavelength of the sky, activates a third type of photoreceptor (not a rod, not a cone) in the retina with a direct linkage to the pineal gland which produces melatonin. When this receptor is activated, it strongly stimulates melatonin degradation. Wearing these weirdo glasses for a couple of hours before bed makes me plenty drowsy, and I've found that I wake up about 7-7.5 hours after I fall asleep feeling fully rested. I tend to go back to sleep for another 30-45 minutes though out of habit. I definitely don't feel like I need that extra bit of sleep.

Having been an insomniac with awful sleep problems my whole life, this is a very, very welcome change. Maybe you could try out the glasses (UBEX brand, amber lens, $10 on amazon) with a nice, comfy blindfold and see if that helps you sleep more efficiently? That seems to me to be the only way to keep melatonin concentrations at a semi-evolutionary level, by blocking out artificial light with the glasses and then ambient light with the blindfold. It worked for me.

My father (grew up on a farm) and grandfather (coal miner) both had "bimodal" sleep patterns, which I always thought was strange until I read more about it relatively recently. They would both fit the definition of "pre-industrial".

My grandfather reportedly used to sleep for a few hours after coming home from work in the late afternoon, then have dinner and stay up late, and sleep a few more hours before going to work. My father used to fall asleep with the kids relatively early, then get up for a few hours, then fell asleep again at around 1-3am. One of my childhood memories is that my father seemed to always be awake, even when I woke up in the middle of the night, or extra-early in the morning.

My entire life I have had problems sleeping more than about 6-7 hours at a time, which again I thought was strange until I read about bimodal sleep recently. I often get very tied at around 8-10pm. If I fall asleep then, I inevitably get up about 3-4 hours later, and then can't sleep again until about 4-5am, then want to sleep for 3-4 hours more. Instead I usually force myself to stay awake until ~11pm and then sleep fitfully for 6-7 hours. I might have a naturally bimodal sleep pattern, which unfortunately doesn't fit well into a 9-5 world.

My sleep cycle could be described as Semiphasic. I normally sleep every 2 nights except for weekends where I sleep for 2 nights in a row about 10-15 hours each, in the week for example Tuesdays and Thursdays I sleep slightly longer.

When I wake up im slightly tired for a few hours, but then I dont feel tired until the late evening or night of the next day and sleep for a good 12 hours, but rarely stay awake for more than 35-40 hours.

When I was a kid I used to sleep every single night but was always tired in the morning and could not get to sleep or get enough, but then I just started staying awake, watching TV or weightlifting, work around the house or college work at night and still had lot of energy the next day, plus it was easier to sleep.

I got used to this patten and it kinda stuck in my brain I guess. Less frequent sleep means staying awake for longer so its not for everyone. But I tend to sleep in big chunks rather than many short naps, stay awake for 35-40 hours then sleep for about 10-15 hours every other day during the week.