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Before an exam, tea and coffee become study tools. A cup at 4 pm, another at 9 pm, maybe an energy drink at midnight. Caffeine does make you feel more alert, but how much is safe for a teenager, and when does it start costing you the sleep that memory depends on? This post covers what food safety agencies and sleep studies say, with the numbers worked out for a typical student.
How much caffeine is in tea, coffee and cola?
Amounts vary with the brand, how strong you brew it and the cup size, so treat these as rough guides. Health Canada's caffeine page lists average values per 237 ml cup (about one large mug):
Brewed coffee: about 135 mg
Instant coffee: 76 to 106 mg
Black or green tea, brewed: 30 to 50 mg
Regular cola, per 355 ml can: 36 to 46 mg
Indian chai made with milk and boiled leaves can vary widely, so if you drink strong tea several times a day, assume the upper end of the range.
How much is too much for a student?
For adults, Health Canada gives a maximum of 400 mg a day. For children and adolescents up to 18, it gives 2.5 mg per kg of body weight per day. The European Food Safety Authority's scientific opinion on caffeine reached a similar adult figure of 400 mg a day, and said the data for children and adolescents were too limited to set a separate safe level, suggesting 3 mg per kg per day as a basis.
Worked example. Take a 16-year-old who weighs 50 kg.
Daily guide: 2.5 mg/kg × 50 kg = 125 mg.
One mug of brewed coffee at 135 mg is already a little over that.
Three cups of brewed tea at 30 to 50 mg each give 90 to 150 mg, so three strong cups can also go over.
A student weighing 60 kg gets 2.5 × 60 = 150 mg, which is about three cups of medium-strength tea.
These are guides for healthy people, not targets. If caffeine makes your heart race or makes you anxious, you need less. This post is general information, not medical advice; if you have a heart condition or take regular medicine, ask a doctor.
How late is too late?
This is where students lose the most. In a home-based study, Drake and colleagues (2013) gave adults 400 mg of caffeine at bedtime, three hours before bed, or six hours before bed, and compared each with a placebo. All three timings disturbed sleep. On the objective sleep monitor, total sleep fell by more than an hour even when the caffeine was taken six hours before bed. The authors concluded that people should avoid substantial caffeine for at least six hours before bedtime.
That was a large dose in a small adult study, so a single cup of tea will have a smaller effect. But the direction is clear: an evening coffee "to finish the chapter" can take away part of the night's sleep, and many people do not notice it.
Does caffeine help you remember what you studied?
There is one well-known lab result. Borota and colleagues (2014) gave adults who rarely drank caffeine either a 200 mg tablet or a placebo after they studied pictures of objects. A day later, the caffeine group was better at telling apart pictures they had seen from very similar new ones. The effect followed an inverted U: more was not better.
That finding is narrow. It was in low-caffeine adults, on one kind of picture memory, and it does not mean caffeine makes up for poor sleep. Sleep after studying is when much of the day's learning is consolidated, which our post on sleep before an exam covers in detail.
What about energy drinks?
Energy drinks often combine caffeine with a lot of sugar, and the caffeine per can varies widely. If you drink them, check the label for the caffeine content, count it in your daily total, and avoid them in the evening. They are not a better study aid than tea; they are just a faster way to reach your limit.
A caffeine plan for exam weeks
Set a daily limit from your weight. Multiply your weight in kg by 2.5 to get a rough daily maximum in mg if you are under 18.
Set a cut-off time. Count back six hours from when you sleep. If you sleep at 11 pm, your last cup is around 5 pm.
Drink it when you need focus most. Use your one or two cups before your hardest study block, not as a fix for a late night.
Keep your sleep fixed. Go to bed and wake at the same times, especially in the week before the exam.
Taper if you depend on it. Cutting suddenly from several cups to none can bring headaches. Reduce by one cup every few days.
If you reach for coffee because a chapter feels too hard to start, a clearer plan may help more than another cup. Fermi's AI tutor works out what you already know and builds a plan for the session, so you can start with the part that matters.
Tonight, add up every cup and can you had today and check it against your own weight-based limit.
