What factors consistently suppress immune function?

Several factors consistently suppress immune function, including sleep deprivation, chronic psychological stress, overtraining, and nutritional deficiencies. Sleep deprivation, in particular, has a profound impact, as studies show that those sleeping less than 6 hours are significantly more likely to develop colds. Chronic stress elevates cortisol levels, which directly impairs immune response, while both overtraining and deficiencies in key nutrients like vitamin D, zinc, and vitamin C can lead to increased susceptibility to infections.