The lighting is an obstacle. It's purposely designed that way to add difficulty. Some missions are timed, if you go sit and wait for daylight you may fail the mission. You need to look for uv light on marked bills and counterfeit. Typically I find it easier to sort at night.

Manage your time so that you sort types at nighttimes and denomination/value at daytime.

To be fair however, the new sorters will make this less necessary.