I had a colleague with this once - she even had medictaion to control it. She would nod off for a minute or so, or even as little as a few seconds, though it was usually longer. Apparently she had no control over it.
I had a similar condition for a while. I found I could not stay awake, would nod off on the bus on the way home, risking missing my stop. On one occasion I even awoke to find my head resting on the arm of a woman who was holding one of those upright bars in the bus sued by standing passengers! I looked around bewildered and she must have thought it was funny as she didn't complain about it at least.
I ws baffled as to what was causing it. I must confess that I don't get much sleep - my fault. I am one of those people who knows they need to sleep but considers sleeping a waste of valuable time in which I could be doing things...like mudcatting.....!! ;-)
So I tried getting more regular sleep and that helped a bit but the problem was still there. Then one Lent I gave up caffeine - all coffee, tea, cola whatever. That meant I also practically gave up sugar, since I used to take one or two spoons per cup (and I drank a lot of tea!). Within a few weeks I found I had a lot more energy, and more to the point - more consistent enegry. There was no more nodding off on the way home or at lunchtime or during the day. Speaking to other people with a similar problem we found the same thing - cut out caffeine as a prop, and regulate sugar intake: hey presto, narcolepsy disappears. (I don't knwo if it will work for everyone, there may be other causes). One root problem seemed to be some people's need (eg. mine) for sugar as a source of energy. This leads to lots of sweet drinks, sugary snacks and chocolate to give energy boost. The boost quickly wears off, leaving you tired again and craving more sugar. The problem is you have no idea how tired and lacking in energy you really are until you stop taking sugar (and caffeine) because your body is being artifically propped up by it.
There'e even a slight 'cold turkey' in the form of headaches, dizziness, irritability (though nothing like cigarettes). But after a few days your body seems to normalise and if you eat food that releases energy slowly, you'll feel fine. I made the latter discovery as a result of mountain walking, where the worst mistake is to scoff food or sugar at the first sign of tiredness. You'll feel fine for about twenty minutes, then your legs will feel like lead. It's better to eat some of that 'trail mix' of nuts or small amounts of dried fruit, or a banana which gives you energy slowly. I usually reserve the main snack and sweet stuff for the summit for a boost before making the descent.
One other thing to watch for - uneven sugar levels and sugar cravings can be a sign of a tendency to diabtetes, so it might be worth getting checked for this if you have a sweet tooth and sudden peaks and troughs of energy.