The reason you cannot sleep may be an evening coffee. How long caffeine actually stays in your body.
Caffeine does not restore energy. It occupies the place where the sleepiness signal would normally land, so you stop noticing the signal. The signal itself keeps building up and returns all at once as the caffeine clears. That is why an afternoon coffee can be followed by a sudden collapse in the evening.
Caffeine takes roughly five hours to fall by half. A cup at three in the afternoon is still half present at eight in the evening and about a quarter at midnight. If you fall asleep but wake repeatedly or sleep lightly, moving your last cup earlier usually helps faster than cutting the amount. Even for the same person, the clearance rate can change with pregnancy, some medications or smoking, so treat this figure as a rough guide only.
Counting only coffee misses your real intake. Green tea, black tea, cola, energy drinks, chocolate, and some headache and cold medicines all contain caffeine. Small amounts through the day often add up to two or three cups of coffee. On days you take cold or pain medicine, check the label for caffeine and cut back on coffee accordingly.
Daily use reduces the alertness you feel from the same amount. Stopping abruptly from there can bring a headache and sluggishness for a day or two. That is not damage, it is the adapted state unwinding. If you plan to cut down, reducing over several days is more comfortable than stopping at once. When a headache hits, before reaching for a painkiller, check whether you skipped your usual coffee time; it often explains the cause.
Guidance from several public health bodies for healthy adults generally puts the upper limit at around 400 mg a day. That is a level known to cause no problems for most people, not a target, and the figure can differ between agencies and over time. The caffeine in one cup varies greatly by shop and size, so counting 'cups a day' easily misses the real amount. Check the stated content of the drinks you have most often and you will get a feel for it. If smaller amounts still affect your sleep or heart rate, that is your own limit.
The same dose feels different depending on timing. A few principles make it easier to get the effect you want from less.
The most common mistake is the loop of covering lost sleep with caffeine and then losing sleep because of caffeine: an afternoon coffee leads to shallow sleep, a more tired morning, and more coffee. To break it, moving the time earlier is usually easier than cutting the amount. Decaf still contains a small amount of caffeine, and treating green tea or cola as fine because 'it is not coffee' is a frequent misconception. Mixing energy drinks with alcohol can make you feel less drunk than you are, which can be dangerous.
A racing heart, shaky hands, anxiety, or trouble falling asleep mean the current amount is past your own limit. During pregnancy, or with heart or stomach conditions, the sensible ceiling is far below the general one, so check with whoever is treating you rather than deciding alone.