How to detect if the user eyes are closed?

Neither the Rift nor Gear VR has any sensor that can give you this information. You would have to make a new device.

Thank you Vrdaveb.
Do you have any leads for this kind of library?

A library isn't enough, you would need to build a new VR headset or use something other than the rift / gearvr. The CV1 does contain a small sensor inside, but it can only tell proximity to your forehead, it can't see your eyes.

The FOVE hmd could do it, it has integrated eye tracking.

Some EEG headbands can read eyelid muscles, but they probably won't fit if you have a VR hmd on.

Thank you Kojack.

Hello OrenFrtidman,
To achieve that you will need that kind of hardware: http://www.smivision.com/en/gaze-and-eye-tracking-systems/home.html