Occupational impact on sleep is mostly associated with psychosocial factors. In particular high work demands (stress) cause increased activation, which causes difficulties initiating or maintaining sleep. At severe levels, characterized by preoccupation with the work situation sleep is strongly affected and may in the long run lead to burnout, with very poor sleep (far beyond that of the insomniac) and extreme exhaustion. Shift work also disturbes sleep and causes fatigue. Physical work factors seem to have only marginal effects on sleep.