A thermal system for enhancing a subject's sleep, the system comprising: a thermal regulator unit having a housing, the thermal regulator unit configured to control the temperature of a thermal transfer fluid; a thermal electric module (153, 155, 163) within the thermal regulator unit configured to heat, cool, or heat and cool the thermal transfer fluid; a first reservoir of thermal transfer fluid within an internal plumbing of the thermal regulator, wherein the first reservoir is configured to be thermally regulated by the thermal electric module; a second reservoir (174, 201) of thermal transfer fluid that is not thermally regulated by the thermal electric module, wherein the second reservoir is configured as a removable cartridge adapted to be coupled to the thermal regulator unit; a valve configured to transfer fluid between the first reservoir and the second reservoir to maintain a level of thermal transfer fluid within the first reservoir; a thermal applicator (300, 402) configured to be worn against a subject's forehead over the subject's prefrontal and frontal cortex, wherein the thermal applicator is configured to pass thermal transfer fluid from the thermal regulator unit; a controller within the thermal regulator unit configured to regulate the temperature of the thermal transfer fluid based on one or more thermal control profiles to modulate the subject's sleep by controlling a temperature of the thermal applicator, wherein the controller is configured to modify the temperature of the thermal transfer fluid at the wakeup time to wake the subject; and a headgear to maintain the thermal applicator in contact with the region of the subject's head over the prefrontal and frontal cortex. a thermal regulator unit having a housing, the thermal regulator unit configured to control the temperature of a thermal transfer fluid; a thermal electric module (153, 155, 163) within the thermal regulator unit configured to heat, cool, or heat and cool the thermal transfer fluid; a first reservoir of thermal transfer fluid within an internal plumbing of the thermal regulator, wherein the first reservoir is configured to be thermally regulated by the thermal electric module; a second reservoir (174, 201) of thermal transfer fluid that is not thermally regulated by the thermal electric module, wherein the second reservoir is configured as a removable cartridge adapted to be coupled to the thermal regulator unit; a valve configured to transfer fluid between the first reservoir and the second reservoir to maintain a level of thermal transfer fluid within the first reservoir; a thermal applicator (300, 402) configured to be worn against a subject's forehead over the subject's prefrontal and frontal cortex, wherein the thermal applicator is configured to pass thermal transfer fluid from the thermal regulator unit; a controller within the thermal regulator unit configured to regulate the temperature of the thermal transfer fluid based on one or more thermal control profiles to modulate the subject's sleep by controlling a temperature of the thermal applicator, wherein the controller is configured to modify the temperature of the thermal transfer fluid at the wakeup time to wake the subject; and a headgear to maintain the thermal applicator in contact with the region of the subject's head over the prefrontal and frontal cortex.