What Causes A Person To Pass Out Immediately Upon Lying Down?

Hi, my boyfriend seems to be experiencing lots of moments where he feels absolutely fine but yet will lie on the couch and pass out instantly. And on days this happens he can sleep all night long - 8pm till 5:45am. Any idea what may be causing this? He does not feel sick at all and is not running a fever, works a day job fine without issues??? Is 39 years old... Help please