Sleep deprivation has been studied for over a century, providing knowledge about the benefits of sleep for many physiological, cognitive, and behavioural functions. However, there have only been anecdotal indications about what a tired or sleep-deprived person looks like, despite the fact that appearance influences not only how other people perceive a person but also how they evaluate them and behave towards them. How someone with sleep loss is perceived and evaluated by others is the focus of this thesis. Facial photographs of 48 participants were taken after normal sleep and after either one night of total sleep deprivation or two nights of partial sleep deprivation. The photographs were then evaluated in four different studies by a total of 288 raters recruited from universities and the general public in Stockholm, Sweden. The faces were rated on attractiveness, health, tiredness, sleepiness, sociability, trustworthiness, employability, and leadership ability. These factors were all adversely affected by sleep loss. Furthermore, looking tired was strongly related to being less attractive, looking less healthy and less trustworthy, and being perceived as a poorer employee and leader. One of the studies assessed facial features commonly associated with looking tired, showing that sleep deprivation results in eyes which appear more swollen and red, with dark circles and hanging eyelids, as well as paler skin with more fine lines and wrinkles. When sleep deprived, people were also perceived as more sad. In conclusion, the four studies show that sleep loss and a tired appearance affect how one is perceived by other people. These perceptions may lead to negative evaluations in interpersonal situations, both personal and professional. This thesis thus demonstrates social benefits of prioritizing sleep, adding to the physiological, cognitive, and behavioural research on sleep loss.