Feeling very tired after mental execises

[Time Traveler]

Recently I'm doing very intensive mental training, like memory training, poker theory, ... I am training my brain really intense, may be first time in my life.

I feel sleepy all the time. I can sleep 8 hours a night and then 4 at day. If I sit to meditate I fall asleep immediately.

Is it normal ?

May be I should use some supplements for brain-work junkies ? 

[Rinne]

This could be homeostasis kicking in. Have you ever pushed yourself this hard? Did you build your momentum up to those things your doing?
Allow yourself rest periods throughout the day (especially after work, school, physical exercise, eating etc etc) if you need them. Simple lay down on your bed and just relax for like 5-30 minutes, you choose. 

[B_Naz]

Try keeping your eyes open when you're meditating. That should stop the sleeping

[Moreira]

Magnesium supplement is the king for the brain.

[Time Traveler]

Interestingly, I feel more capable on physical training. I do my mental exercises together with physical and my endurance increases many times. May be it is just that I turn my focus away from fatigue. 

[d0ornokey]

yeah me too. just a couple hours on business stuff and i'm tired