At low enough environmental noise you might hear your own heart beating, especially when you have one ear against a pillow. I can hear that at around 4am in December or January when I'm staying up in the mountains for a weekend (when and where I don't need the A/C or fan but I'm still wearing socks). Then again, during that time of the year I'm very likely to have had steaks with a side dish of crispy, sauteed random pork bits (chopped face and ears, belly strips, liver, with onions and chili peppers) and lots of beer, plus a few cigars, so my heart's beating a lot harder (walking up a mountain slope, even on a paved street, is still tiring when it's a 65deg incline, even if it's cold and I don't sweat).