Ditto Reamling. 

Also, a dark room helps a lot.  I used to keep the regular lights
on, and found that my eyes like it better when the computer screen is just
about the only available light.  This is especially true if you wear glasses ...
the reflections from the rest of the room hitting the back of the lenses
will distract and add glare, even if you aren't aware of them.

When I feel especially sleepy on one task, it often means I really don't like
this particular task, and I'm sort of hiding the dislike from myself. 

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