When you're standing in front of your sink holding your toothbrush, and you're looking over and cataloguing all the spreadable substances on the counter because you've forgotten what you normally put on the toothbrush to brush your teeth (or even what it's called) because your brain is sleep-fogged to the point of not even being able to run on autopilot, that's when you know it's time to pack it in and call it a day. Even if it is only 10am.
Hair gel? No, that's pink, the stuff I'm looking for is blue... Listerine? No, that's blue also, but it's liquid... Shaving gel? No, it's initially green and it becomes too foamy...