Many years ago I went to India to cycle from Manali to Lei. This was in North Eastern India near the Himalayan mountain range. The highest attitude we did was 17,500 ft above sea level. We have a guide in a 4x4 following us and a lorry that carried the tents, gear, live chickens, and kitchen.

The effects of attitude sickness, varies with different people. You start to get light headed, and if you over exert yourself on steep climbs, you get this feeling of tunnel vision, your breathing becomes difficult, your handlebars will start to wander and your legs start to feel heavy. It feels so surreal, other people talking to you sounds like an echo, like a dream. Rest and lots of water helps but getting down to a lower attitude is the only cure if nausea, vomitting and massive headaches occour. A good night's sleep at a slightly lower attitude with food and warm tea...does wonders to recovery from attitude sickness.