I called this "Chronic Fatigue" when I had it because I couldn't sleep. I presume it's systemic Candida. Take Caprylic Acid to whack back the magnesium-stealing Candida. I had this exact same thing after gallbladder surgery from Antibiotic use, and Dr. Dorman, bless his heart, put me on a NO Sugar DIET! No sugar, no sweet fruits or vegetables, no fake sugars and no yeast breads which have a high yeast feeder Sugar component. I was fixed in 24 hours!
The sensation is like that of normally falling asleep and suddenly finding your consciousness threading down a wrong turn! It's quite disturbing and you jerk awake. This is the leakage of the body's internal feedback system into consciousness that I suspect is present in those with chemical imbalances like Bipolar disorder. It is an alarm system, though! Don't forget that Candida can induce many symptoms. Treat for Candida and you'll perk up quickly. God bless!