Follow up:
> Heard this on the morning news:  They said that cortisol is
> higher in the morning, exercise raises cortisol; therefore
> exercising in the morning gives a double boost.  They claim
> that cortisol suppresses the immune system.
It would seem that the new media are a bit off, as always.
Here's the best I could find:
     Stressors such as injury, infection, inflammation and
toxins will produce       inflammatory cytokines....
     The glucocorticoid (principally cortisol) family
inhibit cytokine production by directly interfering with
immune pathways, thus account for many of the inhibitory
effects on the immune response during stress. They directly
inhibit growth hormone production, contributing to delays in
growth often seen in chronic disease and emotional
deprivation in childhood They suppress hormones associated
with the function of our reproductive organs, contributing
to abnormalities of menstrual and reproductive function.
Inflammatory conditions including asthma, rheumatoid
arthritis, psoriasis and cystitis can be activated by
stress.
     ...To a new challenge, cortisol secretion represents a
healthy response, whereas consistently high cortisol
reactivity to repeated stress results in an abnormal
response that is equally as damaging as too much cytokine
production.

For the full article, see:
http://www.quackbusters.com.au/chronicles/stress.htm

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