Why am I always falling to sleep?

Here's the deal. I'm never tired but when I get board or when something doesn't hold my attention I immediately fall to sleep. I get plenty of sleep at night, I exercise and eat right, I don't have kids, I'm not over weight and I don't feel tired. If I am doing things all day I am totally fine. But long drive's in the car...I get board and start to fall to sleep, watching any type of TV or movies...sleep, non-interesting conversations...mind wonders and I fall to sleep. And its not like I fall to sleep and then wake up 10 minutes later, its 5 minutes I'm out like a light and I sleep for two hours. Example of how fast I will fall to sleep....I sat down to watch 1000 ways to die with my husband and I was sleeping before the first way to die even got started. Whats the deal guys?