Research has shown that getting some sleep is often the way that dogs try to cope with the stress of being left on their own. Analysis of the hormone levels in dogs' urine (from the first time that they urinate after the return of their owner) has shown that, even though they appear to have been sleeping peacefully, they've actually been getting extremely stressed.
So you dog is probably sleeping because he's missing you so much.
Alternatively though, he could just be a lazy little so-and-so ;-)