Injected sclera, or red eyes, can come from many different sources. It is important to differentiate conjunctivitis, or inflammation of the white part of the eye, with injection, which is the term used to describe an red eye from any cause. As your question seems to imply no other symptoms, such as decreased vision, itching, pus, or other issues, we'll address injection first (seek medical consultation if you're having those other symptoms).
Isolated injection of a single eye implies that something caused a local response. It is generally doesn't have any other symptoms and goes away quickly. It is often caused by some sort of minor trauma, and can even occur while sleeping (such as when you rub your eyes or get something in your eye). If your eye has become less red already and there have been no other symptoms, the odds are good that this is what happened. If it happens repeatedly, doesn't improve, or occurs more frequently than in the past, you should have a physician examine your eye.
Inflammation of the eye (conjunctivitis) generally involves itching, discharge, or other eye complaints which sound like more than you had yesterday morning. Such may stem from allergies, infection, or many other things (contact lenses
, dry eyes, too little sleep, etc). If you experience discharge, itching, or especially any loss of vision, you should seek medical attention immediately, as they may be symptoms of a more ominous condition.
Lubricant eye drops may help to some degree, but medical aid should be sought if the condition does not resolve quickly.