Caffeine is a loan against sleep. Know your interest rate.

Quick answer: For most healthy adults, up to about 400 milligrams of caffeine a day, roughly four 8-ounce cups of coffee, isn't linked to harm, according to the FDA. The limit that matters more is the clock. Stop at least eight hours before bed. And if you're already short on sleep, coffee hides the tiredness without fixing it.

Coffee doesn't give you energy

It borrows it. Over the day your brain builds up adenosine, a chemical that makes you feel sleepy. Caffeine sits in the spot where adenosine would land and blocks the signal. The adenosine keeps piling up behind the block. When the caffeine wears off, it all arrives at once.

That's the crash at 3 p.m. It's also why the fix feels so obvious. You pour another cup.

A loan is fine when you can pay it back. Plenty of people can. One coffee at 7 a.m. after a decent night is a small loan you settle by bedtime. Trouble starts when you borrow against a debt you never cleared.

The math nobody does

Caffeine has a half-life of about five hours in the average adult. It varies a lot from person to person. Pregnancy and some medications slow it down. So take the average and do the arithmetic.

You drink a 200-milligram coffee at 3 p.m. By 8 p.m., half of it, about 100 milligrams, is still in your system. By 1 a.m., about 50 are. That's a little more than a cup of tea's worth of caffeine, circulating while you try to fall asleep.

You may not feel it. That's what makes it sneaky. Sleep researchers have found that caffeine taken six hours before bed still cut into sleep time, even when people didn't report feeling wired. You fall asleep fine and get less deep sleep, then wake up a little foggy and reach for the pot. The loan renews itself.

How much is in your cup

People guess low. Check before you decide you're a light drinker.

• A regular 8-ounce home-brewed cup runs roughly 80 to 100 milligrams.
• A big 16-ounce drip coffee from a chain can carry 300 or more. Check the chain's website.
• Energy drinks range from modest to well past a strong coffee. Read the can.
• Cola has a fraction of a cup of coffee, but it adds up across a day.
• Pre-workout powders are often stronger than you'd guess. Read the scoop size and the label.

Count it for three days. Write down every cup, can and scoop. Most guys who think they have "a couple of coffees" find a number they don't love.

Where the debt comes from

Picture a roofer who starts at 5:30 a.m. He gets up at 4:45 after six hours of sleep. Big coffee in the truck. Another at the 9 a.m. break. A can of something cold at 1 p.m. because the afternoon drags. Home at 4, wide awake at 11, asleep by midnight, up at 4:45.

He's not lazy and he's not weak. He's paying 40 minutes of sleep a night he doesn't have, and caffeine is covering the gap well enough that he never notices the bill. Five nights of that's more than three hours behind by Friday.

New parents live here too, and nobody should lecture them. A baby doesn't care about your cutoff time. If that's your season, the move is teamwork. Split the night in shifts so each of you gets one solid block. A husband who takes the 1 a.m. feeding so his wife gets five unbroken hours is worth more than any espresso.

When caffeine stops working

Here's the line to remember. Caffeine fights the feeling of sleepiness. It doesn't fully fight what short sleep does to your judgment, reaction time and memory. You can feel alert and still be slower than you think.

That matters most behind the wheel. Caffeine takes about 30 minutes to kick in. If your eyes are closing on the highway, a coffee at the next exit won't save you in the next five miles. Pull off. Take a 20-minute nap, even sitting up, then have the coffee. Don't drive your kids on four hours of sleep and a Monster. That's a loan your family pays.

The same goes for power tools, ladders and anything with a blade or a load on it. If you're running on fumes, do the safe job and save the dangerous one for tomorrow.

Signs it's costing you

Coffee is probably costing you more than it's giving if any of these sound familiar:

• You can't function, or get cranky, before the first cup.
• You wake at 3 or 4 a.m. and can't get back down.
• Your heart pounds, flutters or races after a normal amount.
• You feel jittery or anxious by midmorning.
• You need caffeine again every afternoon at the same hour.
• You get a headache if you skip a day.

Some of that's ordinary dependence. It isn't a moral failing. It's chemistry. But if the pounding heart comes with chest pain, shortness of breath or lightheadedness, call your doctor. Read what a heart attack actually feels like so you know what to take seriously. Caffeine can raise blood pressure for a time, so if yours runs high, or you're on medication for it or for anxiety, ask your doctor what's right for you. Pregnant women get lower limits from their doctors, so ask yours.

And if you sleep seven or eight hours and you're still dragging, with loud snoring or waking up gasping, say so at your next checkup. Those can be signs of a sleep disorder, and no amount of coffee fixes one.

A plan that works

1. Set a cutoff. Last caffeine eight hours before bed. Bed at 10:30 means nothing after 2:30. Earlier is better if you're sensitive.
2. Cap the dose. Stay at or under 400 milligrams unless your doctor says otherwise. Your own limit may be lower.
3. Move the first cup. Wait 60 to 90 minutes after waking. Many people find the morning dip eases on its own, and the cup lands harder.
4. Replace the 3 p.m. cup. Cold water, a ten-minute walk outside, or a snack with protein. Light and movement tell your body it's daytime.
5. Cut slowly. If you want to drop down, take away a quarter of your intake every few days. Quitting cold turkey buys you a headache.
6. Pay the debt. Protect one night a week for a long sleep. It won't erase months of shortfall, but it helps. Put the phone in another room. If stress keeps you up, it doesn't stay in your head, and it's worth dealing with directly.

What to ask the doctor

Bring your three-day tally. Ask whether your heart rate and blood pressure are where they should be, whether any medication you take interacts with caffeine, and whether your sleep complaint deserves a closer look. It's a ten-minute conversation that most people skip for years.

Coffee isn't the enemy. It's a good morning ritual, a reason to sit down with your wife before the kids wake up. Just don't mistake the loan for the income. Sleep is the only thing that pays the bill.