You pop open your lens case in the morning. The solution from last night is still in there. The lenses look clean. You think about just adding a few more drops and calling it done. Do not.
Here is what happens in that used solution overnight and why pouring it out every single morning is non-negotiable.
Used Solution Is Not Clean Solution
Multi-purpose solution disinfects lenses over several hours. The disinfecting agents are consumed in the process. By morning, the solution has killed the bacteria it can kill, neutralized the proteins it can neutralize, and exhausted its disinfecting capacity. Adding fresh solution on top does not restore that capacity. It dilutes contaminated solution with clean solution, which is not the same as replacing contaminated solution entirely.
Biofilm Forms Fast
Within hours of your lenses sitting in used solution, a biofilm — a thin layer of bacteria and protein — begins forming on the inside of your lens case. Topping off the solution does not remove biofilm. It adds fresh solution to a contaminated environment. Over time, the biofilm thickens. The case becomes a bacteria reservoir. Every night you reuse solution, you are storing your lenses in that reservoir. Replace the case every three months. Replace the solution every night. Read the case cleaning guide here.
The Only Exception: Travel Emergencies
If you are on a plane, in a hotel with no solution, and your lenses need to come out, use bottled water to rinse the case — not the lenses — and store the lenses in whatever small amount of fresh solution you have. Replace everything properly as soon as you can. This is a temporary emergency measure, not a routine.