Although this research was specific to MS* there is reason to believe that cocoa which contains anti-inflammatory components could be generalized to fatigue from other causes.

“Daily use of mugs of cocoa may help people suffering from the common symptom in multiple sclerosis. We are talking about extreme fatigue. Cocoa contains flavonoids, and observation of patients showed that consumption of this drink daily for 6 weeks reduces the level of fatigue and pain. British researchers believe that to thank for this you need the cocoa flavonoids that have anti-inflammatory properties.”

“A new study found that cocoa may be one means of combating fatigue. The researchers divided 40 patients who had recently been diagnosed with multiple sclerosis into two groups. The first used a Cup of cocoa with high levels of flavonoids along with rice milk every day for 6 weeks and the second group is the same drink but with a low amount of flavonoids. Assessment of the level of fatigue showed that it was in the first group, the efficiency of cocoa was the highest.”

https://galpost.com/doctors-told-to-whom-it-is-useful-to-drink-hot-cocoa/10852/

*”Multiple sclerosis is one of the most common neurological disorders, often causing disability. Thinning of the myelin sheath of the nerves through which the brain sends signals to the body, leads to a gradual loss of control over the movements. Many patients after a while are confined to a wheelchair. However, each case of multiple sclerosis is different and for some patients it progresses slowly over decades and others it develops very fast. It is known that this disorder is two times more common in women than in men.”