Microsleep
Microsleep can be extremely dangerous in situations when people drive a car, work on a machine or otherwise need to stay alert.  People with  microsleeps usually are not aware of them. They instead believe that they were awake all the time.  One example is called "gap driving":  a person is driving a car and suddenly realizes that several seconds have passed by unnoticed.  The driver does not feel that she or he has been asleep during those missing seconds, although this is in fact what happened.  The sleeping driver is at very high risk for having an accident during microsleep.