You need to keep warm at sleep,because you still need much calories to keep your metabolism(新陈代谢).Life activities need calories all the time,not just in the day time,but also in the night.And the amylase(酶) in your body just have effct on your life activities in the suitable warm.

你需要在睡觉时保持温暖是因为你在进行新陈代谢时需要热量.生命活动不仅在白天需要热量,在晚上也一样.而且你体内的酶只有在适温下才能对你的生命活动起作用.