Keto!

Purple

I think sometimes maybe

Stefano Zeppilli

Non mi sembra cambi molto. Ci si sente meglio perché si dimagrisce, credo. 

Cici

According to those who've done it (and I'm majorly referring to those who wrote about the diet in the book Tools of Titans) it does the opposite ㅡ that it makes one actually mentally acute. They attribute this to increased ketone bodies amount in bloodstream. 
Also, some say keto diet, it being low carbs, makes them feel alert since they're free from the food coma effect & bloating carbohydrates bring about.
I've seen a person who personally says bulletproof coffee (well known to be keto diet) conteibuted to his alertness.
Based on this I'd say the diet rather does the opposite of making lethargic although more evidence is needed. However, the diet still could make tired by putting much stress with the imbalances in nutrients composition. It'd be more of mental nature. Also, body tries hard to adjust to the new eating.

Wendy Li

Probably at least a drop in energy but eventually your body regulates itself.