One must now consider why some studies have failed
to find effects of caffeine on alertness. For example,
Svensson et al. (1980) found no effect of 100 mg of caffeine
on the mood state of 23 volunteers. Similarly,
Swift and Tiplady (1988) found no effect of 200 mg of
caffeine on the mood of elderly volunteers. This lack of
effect could possibly reflect sample size or other details
of the methodology. Lieberman (1992) suggests that
beneficial effects of caffeine on alertness are most easily
demonstrated when circadian alertness is low and mood
is measured in the context of doing demanding performance
tasks. Rusted (1999) also suggests that mood
effects occur after changes in performance, and this may
account for the absence of effects in certain studies.
Another possible explanation of the failure to find
positive mood changes in certain studies is that they are
masked by increases in negative mood. Indeed, a number
of results suggest that caffeine may increase anxiety
and these are reviewed below.