Because the Yang and Yin Heel Vessels are described as trading dominance between waking and sleep, Chinese medical texts treat an imbalance between the pair as a plausible explanation for sleep disorders, an excess of Yang Heel Vessel activity associated with an inability to settle into sleep, and reduced Yang Heel Vessel activity, allowing the Yin Heel Vessel undue dominance, associated with excessive drowsiness.
This framing gives the extraordinary vessels a role in a category of complaint, disturbed sleep-wake rhythm, that the twelve regular channels' organ-centered model addresses less directly, one of the reasons acupuncture practice retains the extraordinary vessels as a distinct diagnostic and treatment layer rather than folding their function entirely into the regular channel system.
The mechanism the classics offer for this is the daily circuit of defensive qi. In the account given in the Spiritual Pivot, wei qi travels through the yang aspect of the body during the day and passes into the yin at night, and the Heel Vessels are the route by which it crosses. Sleeplessness on that reading is a failure of transfer rather than a disorder of any organ: the defensive qi cannot enter the yin, so it keeps circulating in the yang and the eyes will not close.
Modern acupuncture practice that uses the extraordinary vessels for sleep complaints is generally working from this passage rather than from a separate later theory, which is why treatment tends to address the pair together rather than the yang vessel alone.