Keep the computer screen out of your eyes and you'll be fine.

Look up, "computer vision syndrome". No substantial evidence of "bad" but
screens do cause strain. I think we need to rethink the lighting
arrangements for computers.

There are a lot of health and safety publications about the use of
computer display screens. Most of them suggest that you take short
breaks from using such screens now and then. In the UK I think that
employers are required to take note of the use of screens by their
employees and provide free eye tests, but the response varies between
different employers. However:

1) The studies behind these publications mostly considered reported
eyestrain immediately afterwards, and didn't attempt to measure any long
term effects.
2) The studies were done when computer screens first came into use, and
therefore included e.g. old green-screen CRTs, with dim displays and
obtrusive reflections.

See e.g. http://www.hse.gov.uk/press/2003/e03105.htm, http://www.hse.gov
.uk/PUBNS/leaflets.htm, and http://www.hse.gov.uk/lau/lacs/16-1.htm,
which actually claims no effect as follows:

10 (1) There is good evidence that work with DSE does not cause any
permanent damage to eyes or eye sight, although complaints of temporary
discomfort, eye strain and headaches are common. As with any work that
is visually demanding, users with existing uncorrected vision defects
are more likely to suffer fatigue and stress in DSE work. The eye tests
and corrective appliances provided under reg. 5 are intended to be a
means of alleviating such problems.
(end quote)

I conjecture that modern back-lit LCD displays with a digital feed from
the computer are actually better for you than spending the same amount
of time reading text on paper, because the distance is usually greater,
the contrast is better, and you can adjust text size to suit you.
However, since we don't seem to have 'gold standard' experimental (as
opposed to observational) evidence even for the effects of reading text
on paper, I'm not expecting to be shown correct or otherwise any time
soon. This is a pity, because we now have the option of advising people
to use one or another for large periods of time, e.g. while at school or
college.