burrowing and sleeping in substrate?

its gotten alot colder outside reciently and i moticed that my snake is burrowing under his substrate on the cool side and is sleeping there instead of his hide for some reason... the husbandry is all correct... i've had him for almost a year and hes never done this before but then again i've never had him throught a winter... any suggestions of why hes doing this all of a sudden? should i get him a new hide? anyone else's snkae do this? any input would be apperiated!! thanks