I tested this and it appears that when using the item, you get 50% of your fatigue refilled (equivalent to 6 hours of sleep) and when it expires your fatigue is reduced to 16.67% of its maximum (equivalent to 2 hours of sleep). At least on Pilgrim. I do not know whether it behaves differently on other difficulties.
Edit: also, it does not seem to prevent your stamina or fatigue from decreasing, contrary to the description of the "energy boost" benefit. Sprinting or climbing still drains both stamina and fatigue, the latter at a reduced rate due to the buff which seems identical to the one given by coffee.