I'm not sure what it is, but I've sure been slumbering nicely. So much so that my husband complains that I snore. :) Well, that makes BOTH of us! He's just more sensitive to it. No problem. He just finishes sleeping in the guestroom. In the meantime, I've been so comfortable with my mattress/topper configuration. Even with the chronic sacroiliac issues, I still sleep pretty darn good! Part of it might be because I'm trying to go to bed earlier--by midnight at the latest, preferably earlier if I can. Whatever it is, I'm LOVING it! I just hope it continues.