Caffeine: Benefits, Safe Limits and Sleep

For many of us, the day doesn’t start until we’ve had a coffee. Caffeine is the world’s most widely used stimulant, and it’s found in coffee, tea, cola, energy drinks, chocolate and some supplements. Used well, it can be a helpful tool. Used too much, or too late in the day (or before scheduled sleep for shift workers), it can affect sleep and anxiety.

How caffeine works

Throughout the day, a compound called adenosine builds up in the brain and helps make us feel sleepy. Caffeine works mainly by blocking adenosine receptors, which is why it makes us feel more alert (Fiani et al., 2021).

What the research suggests

• Alertness and attention: caffeine can improve alertness and reaction time, especially when you’re tired. Its effects on memory and higher-order thinking are less clear (Nehlig, 2010).
• Physical performance: caffeine may improve endurance performance in some sports (Ganio et al., 2009).
• Tactical and shift settings: sports nutrition experts recognise caffeine as a tool for alertness in military and first responder roles, used strategically and within safe limits (Gonzalez et al., 2022). A study of special forces soldiers during a 96-hour combat exercise looked at how caffeine use related to both cognitive performance and sleep (Erez et al., 2024).
• Coffee as a whole food: coffee contains polyphenols, plant compounds with antioxidant properties (Butt & Sultan, 2011).

How much is safe?

Food Standards Australia New Zealand (FSANZ) advises:

• Healthy adults: up to 400 mg of caffeine a day from all sources, with no more than 200 mg in a single serve
• Pregnant or breastfeeding: no more than 200 mg a day
• Under 18: no more than 3 mg per kilogram of body weight in a single serve (for example, 120 mg for a 40 kg child)

The caffeine in coffee varies widely. A large or extra-shot coffee can contain more than you might expect, and the Food Standards Code caps energy drinks at 320 mg per litre (up to 80 mg in a 250 ml can). Products with added caffeine or guarana must say so on the label.

Caffeine, sleep and anxiety

Caffeine can stay in your system for several hours, so an afternoon coffee may make it harder to fall asleep or reduce how deeply you sleep, even if you don’t notice it. Some people are also more sensitive to caffeine, and it can increase feelings of anxiety, a racing heart or jitteriness.

If you live with anxiety, PTSD or sleep difficulties, it may be worth experimenting with less caffeine, or having it earlier in the day, and noticing how you feel.

Practical tips

• Set a caffeine cut-off. Many people find it helps to have their last coffee by early afternoon.
• Shift workers: have caffeine early in your shift, and avoid it in the last few hours before you plan to sleep.
• Don’t use caffeine to replace sleep or food. Regular meals and hydration support steady energy too.
• Watch for hidden sources, such as energy drinks, pre-workouts, cola and some medicines.
• Cut back gradually if you want to reduce your intake. Stopping suddenly can cause headaches and tiredness for a few days.
• Try lower-caffeine options like decaf, half-strength coffee, green tea or herbal teas.

See my posts on The Power of Morning Sunlight to set your mind and body and How Diet Affects Sleep for more sleep-friendly habits.

References

1. Fiani B, et al. (2021). The neurophysiology of caffeine as a central nervous system stimulant and the resultant effects on cognitive function. Cureus, 13(5), e15032. https://doi.org/10.7759/cureus.15032
2. Nehlig A. (2010). Is caffeine a cognitive enhancer? Journal of Alzheimer’s Disease, 20(Suppl 1), S85–S94. https://doi.org/10.3233/JAD-2010-091315
3. Ganio MS, et al. (2009). Effect of caffeine on sport-specific endurance performance: a systematic review. Journal of Strength and Conditioning Research, 23(1), 315–324. https://doi.org/10.1519/JSC.0b013e31818b979a
4. Gonzalez DE, et al. (2022). International Society of Sports Nutrition position stand: tactical athlete nutrition. Journal of the International Society of Sports Nutrition, 19(1), 267–315. https://doi.org/10.1080/15502783.2022.2086017
5. Erez N, et al. (2024). Ad libitum caffeine consumption, cognitive performance, and sleep in special forces soldiers during a 96-h combat exercise. Frontiers in Neuroscience, 18, 1419181. https://doi.org/10.3389/fnins.2024.1419181
6. Butt MS, Sultan MT. (2011). Coffee and its consumption: benefits and risks. Critical Reviews in Food Science and Nutrition, 51(4), 363–373. https://doi.org/10.1080/10408390903586412
7. Food Standards Australia New Zealand. Caffeine powders and high caffeine content foods (recommended maximum levels). https://www.foodstandards.gov.au/consumer/prevention-of-foodborne-illness/caffeine-powders-and-high-caffeine-content-foods
8. Food Standards Australia New Zealand. Caffeine (Food Standards Code limits for cola and energy drinks). https://www.foodstandards.gov.au/consumer/prevention-of-foodborne-illness/caffeine

Disclaimer: While moderate caffeine consumption can offer potential health benefits, excessive intake or sensitivity to caffeine may lead to negative side effects such as jitteriness, insomnia, and increased heart rate. It is recommended to consult with a healthcare professional or nutritionist regarding individual caffeine tolerance and the suitability of its consumption based on personal health conditions.