Exactly how much caffeine will wake you up - but not lead to a crash, and when to drink it, according to experts

Many of us need caffeine to survive the day, but too much can leave you tossing and turning at night and jittery during waking hours.

The official recommendation from the FDA is no more than 400 milligrams of caffeine daily for adults, equating to around four cups of coffee.

But how can you optimize your caffeine intake to get the most out of it?

Around 100 to 150 milligrams, equivalent to one or one and a half cups of coffee, is a rough amount that will give a healthy boost, said Astrid Nehlig, an emeritus research director at the French National Institute of Health and Medical Research.

Nehlig said you will start to feel more awake about five minutes after drinking coffee, and the effects will increase to reach their peak between roughly 15 minutes and two hours later, depending on if you've eaten and the speed of your metabolism.

But too much can lead to negative effects.

A study published in the New England Journal of Medicine last year found that when participants drank as much coffee as they wanted, they slept an average of 30 minutes less than on days when they didn't have any coffee.

Caffeine's impact on sleep depends on how quickly you metabolize caffeine, Gregory Marcus, a cardiologist and professor of medicine at the University of California, San Francisco and first author of the NEJM study, told The Wall Street Journal.

It can take up to ten hours for your body to completely get rid of caffeine.

But genetic differences mean some people metabolize it slowly or quickly, experts say.

The NEJM study found that slow metabolizers slept nearly an hour less when they had coffee, while fast metabolizers saw no change in their sleep.

Experts recommend holding off on the first cup of coffee until from 9.30am to 11am in order to reap the top energizing benefits and avoid the jitters.

Studies show cortisol levels — the main stress hormone — are highest when we wake up, and that having a coffee too early boosts these even more, leaving us at risk of unnecessary jitters.

Studies dating back to 2009 show cortisol levels are highest in the morning and drop throughout the day, with two major dips at roughly 9.30am and 1pm.

Amanda Maucere, a sports nutritionist in Florida, previously told health website Well and Good: 'Although there hasn't been much research done in humans on the interplay between coffee, cortisol, and the circadian rhythm, we know enough about each one separately to suggest that waiting a little bit before having that first cup of coffee in the morning is a good idea.'

She added: 'Although helpful for "waking up", elevated levels of cortisol are also found in times of stress to be harmful.

'So, if you combine an already elevated cortisol with an additional spike from coffee, you create an unnecessary stress response in the body.

'This response would be mitigated by waiting an hour or so before enjoying your morning cup of joe.'

If you're flagging at work in the afternoon, you might be tempted to opt for a tea, which contains about a quarter of the caffeine in coffee.

While this might power you through to the end of your shift, it can also make sleeping that night harder. The reason is too much caffeine disrupts sleep by blocking receptors for the hormones adenosine and melatonin.

Six hours before bed is the optimal time to cut off caffeine, or 4pm for those who try to sleep at 10pm.

The Food and Drug Administration (FDA) says these effects should ease within four to six hours.

But some papers — including one from 2013 — suggest you may need to cut off the drug even earlier to ensure a good night's sleep.

Some experts suggest drinking your final caffeinated beverage at 4pm, while others say you should stop at 2.

Dr Dierdre Conroy, a sleep expert at Michigan University, recommends stopping drinking coffee eight hours before bed - 2pm, for those who go to bed at 10.

She said: 'People have very different sensitivities to caffeine and thosse who consume caffeine more often might respond differently to those who don't drink it at all.

'But in general, our guideline is eight hours before going to bed, you should eliminate all caffeinated products.'

A meta-analysis from 2017 found drinking coffee too late in the evening disrupts slow-wave sleep — the most restful period — and reduces total sleep time.

Not getting enough sleep has been linked to a whole host of health issues, including heart disease, kidney disease, high blood pressure, diabetes and obesity.

The Centers for Disease Control and Prevention recommends everyone gets eight hours of sleep a night, but polls suggest one in three adults don't make this threshold.