Can seasonal allergies cause a severe form of fatigue?

Probably not: Allergies can cause some mild tiredness or fatigue, particularly if they cause trouble breathing at night that prevents a good night's sleep. If you have severe fatigue (trouble walking more than a few steps or getting up out of bed, e.g.) or sleepiness (falling asleep at work or while driving), it is not likely to be allergies and you should see your doctor right away to rule out other causes.