Meditating with eyes fully shut or half shut?

I've recently been experimenting with meditating and performing internal exercises with eyes half-shut, instead of eyes fully shut.

Some meditation teachers emphasize that eyes should be fully shut while meditating to reduce all distraction and retain full attention internally. Others emphasise half-shut eyes, in order to maintain added alertness, and also to avoid going into a 'void' state.

IME, after practicing for years with eyes fully shut, I have come to appreciate the half-shut position more, and now only use this while meditating or performing internal looking.

The reason for this is that IME I found that it not only helped with the above (added alertness, stopping 'void' drifting etc.), but also that it helped with posture and alignment of the head/neck.

It seems that the movement of the eyes definitely affects the position of the head/neck, even slightly or very subtly.

This has been noted in various therapies/systems, such as the Alexander Technique- 'the head follows the eyes, and the spine follows the head' etc. (paraphrasing here).

What seems to be a small point, and is usually brushed off as trivial by most meditation teachers, is actually quite a large point and can have a definite impact on a practice.

Experiment with both, and note the differences.