I think there are a lot of reasons for this, foremost being that it is a fairly restrictive, yet comfortable position to lay in. I am someone who likes to be totally covered up, aside from my head, when I am sleeping, so the coverage of the sleepsack is a perfect alternative to blankets.
Additionally, I love the feeling of waking up, with the tight rubber all around, and my arms comfortably yet securely in the sleeves of the sack. Theres something very comforting about the entire situation, as well as sexually stimulating.
For me, sleeping in a sleepsack is not entirely a sexual experience -- admittedly i am far more likely to get into the sack for bed if I am horny. There are a lot of theories about why the squeeze of the latex is comforting for many people.
For more information regarding this, read about trance suits, or hug suits -- often for autistic patients, as a treatment: http://www.psychforums.com/autism/topic23075.html. I suspect the effect it has on me, is the same soothing and calming effect as it does on those patients.
How do you last 5-10 hours in a sleepsack?
A good question, I havent been able to last 8 hours as reliably as i could when i was sleeping in the sack more, but there are a few key points:
- Lube your arms and shoulders well, this is where problems often arise
- When getting in the sack, make sure the shoulder seams of the sack are even with the middle of your shoulder (this can be HARD on rubber sacks, but its IMPORTANT)
- Choose a sack design with a large enough foot area, what starts out tolerable can soon become extremely painful
- Control the temperature of the room, for me around 66 degrees tends to be comfortable while in a sleepsack for a long period
- Stack your pillows so that you have 'options' as to head angle and placement (this is hard to do once you are zipped in the sack)
- Be calm, enjoy the sensation, close your eyes and feel the experience -- once you can enjoy the sensation, you have a much better chance of achieving sleep
- Practice -- It took me more than a year of sleeping in a sleepsack 2-3 times per week to get to where I could truly get a good nights sleep (Chicagogear can vouch for this, he helped me in most of those nights)