Extreme environments such as high altitude represent a challenging testbed for both people and instruments. During a recent expedition to the slopes of Mt. Everest, (Highcare Expedition), we monitored ECG, respiration, oxygen saturation and posture in five climbers during sleep. Data were collected by using a recently developed textile-based wearable system (the MagIC system). The polysomnographic acquisitions were performed at sea level in indoor environment, and at 6000 and 6800 m asl in tents. The collected data allowed us to investigate performances and usability of MagIC system as a polysomnographic device at very-high altitude. In this paper we report on the results of this evaluation.