You know it's time to sleep when 4 cups of coffee and a Redbull don't lift the weights off your eyelids.
I realise you can be physically tired without being mentally tired but you can't be mentally tired without being physically tired. The mind drains the entire being.
Physical tiredness is easily dealt with. When you're physically tired, you sleep.But when you're mentally tired, you think sleep would do the trick but it never really does. Sleep only gives you the physical energy to deal with your mental tiredness, which tricks you into believing that you're okay when in fact, you're still mentally drained.
Maybe this is how mental illnesses begin.