The effects were studied of an electrically heated blanket (38 W), placed on a sleeping mat, upon the thermoregulatory responses during night sleep at an ambient temperature of 9C. The major findings were that, when the subject used the heated blanket, decline of rectal temperatures was strongly inhibited during night sleep, rectal temperatures being kept at considerably higher levels, compared with those when the subject used the non-heated blanket. The reasons for such diffrences occurring and their implications are discussed in terms of thermal and circadian physiology.