Tom ideally wants to sleep 8 hours a night for 7 days, 8 x 7 = <<8*7=56>>56 hours sleep total for the week.
Tom has actually only slept 5 hours each weeknight, 5 x 5 = <<5*5=25>>25 hours of sleep.
Tom has only slept 6 hours each night on the weekend, 6 x 2 = <<6*2=12>>12 hours of sleep.
Total, Tom has gotten 25 + 12 hours = <<25+12=37>>37 hours sleep.
The difference between how many hours Tom would like to sleep, 56, and how many hours he's actually slept, 37, is 56 - 37 = <<56-37=19>>19 hours of sleep that Tom is behind on.
#### 19