Rating: 3 / 5
As a tall (6'2") and finicky sleeper, I decided to indulge myself by taking the Thermarest Prolite 4 Large sleeping pad on a recent week-long hike in the Grand Canyon. This pad was the heaviest single item in my pack, but if it contributed to more pleasant nights, it would be well worth the weight. Darkness in the Canyon during March extends for 12 hours; that can be a long time if you're twisting and turning uncomfortably. The Prolite 4 was indeed extremely comfortable -- initially -- but every night I would awaken after about 4 hours to find that the pad had lost much of its inflation. At first I thought I had a pinhole leak somewhere, or that the valve was leaking. However, my son, who was using a smaller, Prolite 3, was having the same problem. Our hiking companion was using an older model Thermarest, one that is somewhat heavier, and he was not having this problem. I began to use some of those long night hours to speculate on what might be happening. While I don't know for sure, I was left wondering whether the lighter-weight Prolite series of pads were relying more on air and less on foam as a way of saving weight. With a greater reliance on air, the pads become more susceptible to changes in air temperature, and therefore air pressure. These changes can be quite significant in the desert, especially in March. Thus, when I inflate the pad at 6 pm, the air I blow into the pad might well be 65 to 70 degrees. But 4-5 hours later the ambient air temperature plummets into the 40's. With that drop in temperature comes a corresponding drop in pressure. Thus, as the air inside the pad loses temperature, it also loses pressure, and the pad loses some of its inflation.
If someone out there has had similar problems, or can either verify or discount this theory, I would be very grateful. My back especially.