Well that sensor is going to be placed on a sleeping mask. the distance is in mm so i would say off the top of my head as if your wearing glasses if that makes any sense
Ok, that is a good distance. In any case I would go with an ADC version of a reflective sensor as it will be probably easier to calibrate (auto calibration in absence of different values during a longer period) and trigger the event. Or even to record the signal the whole time and do a post analysis.
Maybe you should also consider to make the IR Diode tunable with a potentiometer (not with PWM as that would interfere with the recording) so while the person is wearing the mask you tune the Diode that you receive a medium value in the photo transistor.