Not all lenses can be worn over night. Make sure you use the right kind and ask. I stopped using sleep in lenses as they are not healthy and generally lenses stop the oxygenation of the cornea which can be damaging in the long term. So use glasses as often as you can, ie getting home etc. You can use small amounts of all-in-one solutions (cleans and stores over night) in the BPL products recommended above or just cannibalize an eye drop bottle or similar (eg: small lens cleaner bottles, can store contents elsewhere at home). Visit a drug store if you're in a hurry to get one. Saliva can be ok if you have to take them out or they just fell in some dirt and you don't have solutions on you. But before putting them back in they have to be cleaned (in water at least properly) as saliva contains many bacteria. Water makes them hard sticky and painful. Wash hands before contact or cleaning.
In general you might want to skip on a fraction of an ounce here or there which is funny when most are overweight. Being under weight or not eating properly generally and specially on the trail is worse and is similar to carrying a heavy load and being miserable on top. In all these cases you will ruin the quality of your days and nights and in this particular case could end up with an eye infection. For short sightedness specially in extreme cases Lasic laser correction which is about the cost of lenses/glasses over a decade is not a bad solution. That is if you are over thirty and the eyes haven't changed much and have stabilized, getting significantly better or worse, in the last few years. Although my eyes are not too bad, I'm looking into it soon.
I've realized a little common sense is usually the answer in almost all situations.
