Q: keyboard and trackpad non responsive at startup

Every now and then when my mbp wakes from sleep, keyboard and trackpad becomes non responsive so I cannot move my pointer around.

Although the computer opens up with the cursor blinking at the password window, keyboard is also inactive and I cannot type my password and login.

I've noticed that the computer is working normally (I guess) in the background because e-mail and calendar notifications are working and showing at the login screen (without mobbing able to login),

The only thing I was able to do to solve this was to "force restart" (long press on the power button) until the computer boots up again and then everything is back to normal.

Needless to say this is very annoying and i couldn't find any mention of this phenomena here or elsewhere.

MBP is mid-2012 machine 2.9 GHz Intel Core i7, with 8 GB 1600 MHz DDR3.

Operating system is El-Capitan (10.11.1) although I should mention I'm suffering from this thing for quite some time, even before upgrading the operating system and before replacing my HD to flash a few months back.

Any thoughts?

Thanks