Insomina is the condition where you have trouble getting sleep.  It can take on a number of forms, from being not all that serious to potentially deadly, if you have a condition like sleep apnea.  A series of sleepless nights can lead to a whole host of problems during the day, from weight gain to lack of concentration.  It can be deadly in an indirect way.  If you are not getting the sleep you need each night, when you get in your car you might not be able to concentrate as well as you should, and this could cause an accident.