What Causes Hardening Of Areoles When Nipple Gets Hard?

For the past two nights, when my left nipple gets hard, occasionally the upper part of the areole will get hard also. It goes away when my nipple returns to normal, and I only happens at night. It s also only on the areole and not anywhere else on my breast. I m 23 and not currently breastfeeding, so I m just a bit curious as to what this could be.