10. Sleep and Fatigue . Inadequate testosterone can also lead to poor sleep quality, which means that the only thing you'll want to do under the covers is put your head on the pillow and crash. Testosteorne deficiencies decrease the all-important recuperative REM sleep and other critical sleep parameters. One study showed that low T did this not only by increasing weight gain, which negatively impacts sleep in and of itself, but also through other mechanisms that we do not yet understand.  And none of this should be a surprise considering how much that testosterone affects the brain.