Coffee is among the most consumed beverage in the world — up there with water and tea — and it contains over one hundred different substances, including fats, minerals and vitamins. But the fundamental star element in coffee is caffeine: a drug that scientists categorize as a “central nervous system stimulant,” and the most widely consumed psychoactive substance on the planet.

Caffeine is the reason why coffee is touted for its effects on energy, focus, memory and fighting off that creeping snooze. Research has shown that coffee can help boost your athletic performance, just a tad. Consuming it regularly throughout your lifetime is also associated with the prevention of cognitive decline and reducing the risk of conditions like Alzheimer’s disease.

But what point in the day is the best for taking a sip?

How Much Coffee Is too Much?

Chug your coffee and you’ll start feeling the effects anywhere from 5 to 45 minutes in. Then, the caffeine has a half-life of 2 to 5 hours. This means that if you ingested 100 mg of caffeine at 10 a.m., you’d probably have up to 50 mg of it still circulating in your bloodstream by 3 p.m.

Caffeine and Your Body

But how your body takes in that caffeine also hugely varies according to your age, sex and hormones, genetics, personal tolerance and lifestyle. Some people are more tolerant of caffeine than others just by nature. Others have developed a tolerance because they’ve consumed a lot recently. For example, researchers have spotted that some people have genes that allow them to metabolize caffeine much faster than others, but they’re also those more likely to consume more coffee as a result.

Caffeine and Anxiety

A moderate caffeine intake in one sitting is anything from 50 to 250 mg, according to the International Scientific Association of Coffee. This can increase your feeling of well-being, relaxation, vigilance and concentration, as well as decrease pain in headaches and migraines. Consuming more than that, anything from 400 mg in one sitting upwards, can easily trigger side effects of caffeine, such as nervousness and anxiety, aggressiveness and tremors. This also happens if you’re one of those people whose naturally more susceptible to the drug.

Read More: The Biology of Stress in Your Body

Caffeine and Sleep

Caffeine has the biggest impact on sleep. The longer we are awake, the more we need sleep. As the day progresses, there’s an increase in the chemical adenosine in the brain, which contributes to making you feel more and more sleepy.

“What caffeine does is block the adenosine receptor, so it kind of blocks that sleepiness signal that you get normally,” says John Stuart O’Neill, a circadian biologist at the MRC Laboratory of Molecular Biology.

Circadian Rhythm

Another element regulating how tired we feel is the circadian rhythm, which is the natural biological clock that’s telling us whether to sleep or be more awake at an expected time of day. We all run on a more-or-less 24-hour biological clock, and studies have shown that coffee-induced arousal, during a time when your body is naturally winding down, can throw off your circadian rhythm.

“When you have caffeine after 5 or 6 p.m., it can signal to the circadian clock in the same way that seeing light at the wrong time of day can,” says O’Neill. “It’s like being jet-lagged.”

His research has shown that all else being equal, caffeine can delay your circadian rhythm by about 45 minutes. “There is a 12-hour window of your day when it is advisable to avoid caffeine,” says O’Neill. “From 6 o’clock at night to 6 o’clock in the morning, these are not good times to be consuming caffeine.”

Avoid Coffee Late at Night

Of course, everybody has an occasional evening coffee if they must finish a project or drive a long way home. That’s not going to completely screw up your sleep hygiene. But making a habit of an evening coffee could potentially mean you’re continually out of whack.

“If your biological clock is continually delayed relative to the natural world and you’re continually getting, say, an hour or two less sleep than you need, the long-term health prospects for you are less good than if you hadn’t done that,” says O’Neill.

Read More: How to Recover From a Sleepless Night

When to Drink Coffee

There’s also research working to show that if you have caffeine too early in the morning, it may advance the circadian clock a little bit so that you would start to feel sleepy earlier. But the effect of that seems to be minimal so far. In fact, there doesn’t seem to be any solid evidence that one time is better than another when it comes to morning coffee consumption.

Drink Coffee in the Morning

“The best time to drink coffee is in the morning,” says William R. Lovallo, professor of psychiatry and behavioral sciences at the University of Oklahoma Health Sciences Center. That’s because caffeine also stimulates the brain chemical dopamine — sometimes called the reward chemical — giving us a little lift and mood boost to get the day started off right. This is also in line with our circadian rhythm, and with the ebb and flow of other hormones that our body produces.

Cortisol

For example, in the morning we also get a little spike in levels of cortisol, a stress hormone that helps our system get going as the sun rises. There have been speculations about whether waiting for the cortisol levels to dip before having a coffee could be a more efficient way to intake caffeine, making the most of the natural burst of energy from cortisol first. But there doesn’t seem to be a lot of scientific evidence to back that up just yet.

“Although cortisol is elevated in the hours after we wake up, that does not seem to mean that it’s not a good time to drink coffee,” says Lovallo who has specifically studied the impact of caffeine on cortisol levels during the day. “It’s not relevant,” says Lovallo. “After all, most people prefer their coffee in the morning, regardless of cortisol levels.”

What’s more, when it comes to the alertness effects of caffeine, don’t forget the placebo effect. Research has shown that people who think they’re consuming caffeine might experience just the same boost as if they had a taste of that sweet stimulant.

Read More: 6 Ways To Wake Up Feeling More Rested