Needless to say, the humans face a number of physiological challenges. According to a Reuters report published today, "[Bertrand] Piccard and Andre Borschberg have been taking turns piloting the plane on each leg of the journey. Both have trained to stay alert for long stretches of time by practicing meditation and hypnosis."
The full article: http://www.reuters.com/article/us-solar-airplane-idUSKCN0YD03H.