Despite attracting increased attention from clinicians, brain temperature distribution, fluctuations, and changes in response to external stimuli are still largely unknown and refractory to conventional temperature-probing approaches. Fluorescence nanothermometry in the second near-infrared window (NIR-II, 1000-1700 nm), as shown here, can provide contactless brain thermal sensing through the scalp and skull in real time with a subdegree resolution. As a proof of concept, Ag2S nanothermometers have been used to monitor brain thermoregulation in a cooling/heating process and temperature changes associated to a barbiturate coma.