Mine has done that alot as well, i just blocked up his cave so he couldnt get in it, but when i take him out and let him rome in the house he always finds a dark spot to lay in. It could be cause of brumation, could just be temporary, i dont think it's anything to worry about, now if he or she starts looking a little sluggish, and doesnt move much then maybe he is constipated and needs a warm bath, just keep a close eye on him or her. Thats the best i can do.