Night Tornadoes are particularly deadly

The only thing more frightening than the sight and sound of a tornado approaching is one that strikes while you're asleep. It comes in the middle of the night, enveloping your neighborhood with dangerous winds, swirling debris, and possibly deadly consequences. 
Nocturnal tornadoes, as they are called, are like nightmares that have come to life. They strike under the cover of darkness and are often among the most deadly weather phenomenon. 

Nocturnal tornadoes are particularly dangerous for several reasons," says Dr. Greg Forbes "One is that it’s nearly impossible to see the tornado coming, so getting and heeding warnings is crucial. Another is that people are often asleep and caught unaware. People are usually at home, often in structures that are not as sturdy as their place of work. Mobile homes are particularly vulnerable.

According to a study done by Northern Illinois University, the fatality rate from nocturnal tornadoes has actually increased in the past century, while the fatality rate from daytime tornadoes has decreased. Researchers found that tornadoes during the overnight period are 2.5 times more likely to kill as those occurring during the daytime hours

http://www.weather.com/news/tornado-central/night-tornadoes-deadly-20130318#