Operating in the hover by night at around 10,000 amsl, no extra O2, observing a ground target.
Suddenly noticed the orange sodium street lights had become monochrome greys. Due to good (military) training, it was recognised as the onset of mild hypoxia. We remained on task at it was an important one, with a crew patter set up between us monitoring my performance to ensure there were no other effects. After a few minutes, job done, we descended and the effects quickly disappeared.
