Here in Devon this week, as in swathes of the UK, it's cold with persistent rain or heavy showers.

In such conditions, many of us spend much more time than usual sitting in front of a PC, tablet, mobile or TV, normally a combination of these over the course of the day. And because it's cold and we are spending more time sitting still, we are likely to turn up the thermostat on the heating system.

The trouble is that our eyes are not designed for spending hours on end looking intently at...