The first and most obvious reason caffeine may not impact you is sleep deprivation. Indeed, research demonstrate that coffee does not boost alertness if you sleep five hours three days a row.
You may also have genes that reduce caffeine's effects. Adenosine receptor variations can prevent coffee from energizing you, according to research.
A caffeine tolerance may also prevent you from feeling its effects. This can happen if you drink lots of coffee daily.
If drinking cup after cup of coffee or tea doesn't affect your energy, you may be dehydrated.
You may also be deficient in caffeine. Getting adequate sleep every night may help, but one cup of coffee doesn't ‘work.’
If coffee is useless after lunch, you may have a midafternoon slump.
Finally, frequent drugs may affect how caffeine affects you.