I used to drink a diet soda with lunch. Then I switched to decaf diet soda for lunch to eliminate the caffeine. Did this for a few months and slept better at night. Then I switched from decaf diet soda to sparkling mineral water to eliminate the Aspartame and I noticed how much better I functioned in the afternoon. It was amazing how foggy-headed the Aspartame was making me. Sometimes I would give in a have a diet soda but only if I knew I didn’t have any tasks to do that were mentally strenuous, because I was more focused and could think more quickly when I avoided Aspartame.
Now I try to avoid Aspartame in everything. It is even in yogurt!