The biggest determining factors for my quality of sleep in descending order are:

1. Noise

   This one is difficult to fix. I've almost always had some combination of noisy neighbors, high street noise (living in a city center or by a highway), loud wildlife (coyotes, etc), and loud roommates. If I fix it once, the move or condition is usually temporary, but man, does the sleep improve while the silence lasts. I have tried custom earplugs (best solution so far), but constant earplug use leads to inevitable ear irritation and an increased probability of infection. I would like to address this in a future project.

2. Light

   Even the smallest sliver of light peeking through a blackout curtain, or leaking through the nose around a sleep mask is enough to wake me up. The best remedy I've found for this is a combination of good sleep mask and blackout curtains. However, no sleep mask has fully met all my requirements, as I find the strap itself uncomfortable. This will be a future project.

3. Temperature

   This is focus of this project. By precisely controlling the temperature of the room and the bed, time-to-sleep and sleep quality are drastically improved.

4. Bedfeel: stiffness

   A good mattress can help significantly. There is a fine balance between too soft and too firm. I am currently using a traditional Japanese cotton futon, but I don't like the hard sewn-on bits, so I will add a foam pad. I've found a good foam topper to be just as good as a fancy mattress. I don't think I would recommend the futon for most people.

5. Bedfeel: softness

   Don't buy cheap sheets, just don't do it. It's not worth it. Ikea is banned. I have tried silk, but it takes too much energy to take care of. Linen is too rough. I prefer high quality softened cotton or Lyocell fabric (woven plant cellulose, as soft as silk--the new production methods are much more sustainable).

6. Stress levels

   (Also related: social isolation)

   This is also difficult to control. Stress has always been present in my life and I assume it is always present in everyone else's. Managing stress is different for everyone.

7. Pillow shape

   A pillow that is too tall or flat negatively impacts sleep, and depends on your mattress stiffness and shoulder size. Generally I've found synthetic pillows to be less comfortable than down, but if you have bad allergies you have to compromise.

8. Exercise

   I find I usually get worse sleep after exercise because I'm aching. I also don't sleep well to begin with, so I can't recover, so I just get even more exhausted than normal. There is also a fine balance between too much exercise and too little.

9. Sleeping position

   Back and side sleeping only, stomach sleeping is banned.

10. Waking time

    If you job allows it, the ideal waking time for me is around 9:40am.

11. Air quality

    A clean mattress and bedroom. A took a blood test and re-confirmed that I have a terrible dust-mite allergy, to the point if I sit on a dusty couch or mattress, I will feel physically ill after a few minutes. I now make it a life rule to never sleep on an old mattress, and to use anti-allergy covers when available, but only if they aren't too stiff. An air purifier may have also helped a bit, if only from the white noise. Actual benefit is questionable.

12. Diet

    Basically, don't eat late, and don't take caffeine.