Coffee doesn't work when we're well rested: sleep researchers find out when caffeine really wakes us up. Heavy drinkers react much worse to the awakening caffeine than people who rarely drink coffee.
If you have a cup of coffee once or several times a day, you promote a habituation effect in your body. At night, if we don't drink coffee, a mini-withdrawal can occur. The resulting tiredness can be overcome with a morning coffee, according to the DGSM experts.