I've certainly had the problem of distinguishing between the two in the past. I would wake up after a nightmare about crashing my car and leaving the scene, and be in a sweat wondering when I'd be caught. It would take me several minutes to realize that I'd dreamt the whole thing. After a few of these, I made a rule that if I were wondering whether I had dreamt something or not, if I was lying in bed when I was wondering about it, it was pretty certain I'd dreamt it.

It sounds crazy.  But I was also working nights and over 100 hour weeks at the time, and I was extremely sleep deprived.

Even mild sleep deprivation does some remarkable things to cognition. Severe sleep loss does make you a bit irrational at times.

ETA: For the lucid dreamers, isn't that why you need to carry your "token" (as in Inception) with you? ;)

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.