Not to make you feel worse, but falling asleep at work can be caused by a health issue, for example sleep apnea. In the future I would recommend approaching this with your employee as a health issue first rather than as a laziness issue.
Manual labor is usually paid hourly with bonuses for overtime. Unless you're keeping a close eye on the workload to make sure not only the entire team but also each team member is treated fairly, you can expect people to do things that resemble slacking off, to manage their own workload. If you randomly require the whole team to work weekends, the person who put in a solid 40 hours during the week loses out compared to the person who left early when they could. If the whole team is sometimes required to stay until 6 or later to fix some crisis, the person who shows up at 9 sharp every day loses out compared to the person who drifts in around 9:30 or 10.
My Dad is straight up the biggest work-a-holic I've ever met. Constantly works 12-16 hour days, weekends, etc. Says he loves it. Came out of retirement to work more. I don't understand it but he's happy.
I got some of my friends jobs at work through him and it got brought up to me that he fell asleep at work quite a lot. Was hard to hide because he snored really loud. Was pretty embarrassing to me. Either way, long story short, he had exactly what the other poster was talking about sleep apnea. Got diagnosed, got a c-pap(?) device and guess what, no more falling asleep at work. Probably saved his life as well, as he's already had a heart surgery.
Either way, I pretty sure he's the most non-lazy person I've ever been around. :)
