Your brain is an energy hog. It takes an enormous amount of energy to listen intently for a long period of time. One secret to keeping your audience glued to your pitch or presentation is to keep your presentation short. But how short? TED Talks have found the answer--18 minutes.

It doesn't matter who you are, how much you're worth or how famous you are. From Sheryl Sandberg to Pope Francis, anyone who speaks at the annual TED conference must follow the 18-minute rule. No presentation may run longer. No exceptions. The TED conference has over 30 years of experience with presentations, so it should know what works and what doesn't.

In TED Talks, TED curator Chris Anderson says that 18 minutes is "short enough to hold people's attention, including on the internet, and precise enough to be taken seriously. But it's also long enough to say something that matters."

Recent research in brain science supports Anderson's conclusion. Learning is draining. The average adult human brain weighs only about three pounds, but consumes an inordinate amount of glucose, oxygen, and blood flow. As the brain takes in new information, millions of neurons are firing at once, burning energy and leading to fatigue and exhaustion.

Cognitive load makes it harder to say yes.

Paul King at Texas Christian University, an influential scholar in the field of communication studies, says the accumulation of information results in "cognitive backlog." It's like asking your listeners to hold more and more weight as you pile it on. Soon, they'll drop it all. 

Think of it this way: If you're giving a sales pitch to a new customer and it's mildly interesting, a five-minute presentation produces a relatively small amount of cognitive load. Your customer is likely to be engaged the whole time. 

An 18-minute sales presentation will add more weight. But if it's well-crafted, offers a solution to a real pain point, and is delivered by an energetic and engaging speaker, it will keep a customer's attention. After 18 minutes, however, attention will drop off significantly. Even the most skilled and dynamic speaker would have a hard time keeping anyone's attention for much longer. It's simply too much information at once.

Pitches are stronger with the laws of subtraction.

I know what you're thinking. "I have too much to say!" Yes, you do, and that's why it's critical to condense, refine, and edit your content. Chris Anderson says most speakers on a TED stage are used to speaking for 40 minutes or more. It's hard to edit, but the presentation will be much stronger because of it.

It's called the Law of Subtraction, the act of removing anything that's excessive or unnecessary. The content that remains is much stronger for it. Unrestrained freedom to talk as long as you want or to create as many slides as you desire will only result in a presentation that is long, boring, meandering, and confusing.

After hearing me speak about the 18-minute rule, a sales director for a large television cable network added a twist.  He now starts a meeting by saying, "I know you've scheduled an hour, but I'll only take about 18 minutes." The sales leader keeps the content to 18 minutes, followed by a question-and-answer period. He says it never fails to put a smile on a customer's face.

The 18-minute rule works and there's science and evidence to back it up. But there is a way to keep the listener engaged for a longer period of time. It requires building in "soft breaks." These include: videos, demonstrations, hands-on activities, inviting other people to speak, or engaging in Q&A with the audience.

When sales professionals ask me how a pitch should last for a first customer meeting, I recommend no more than 18 minutes of content, followed by a demonstration or interaction with the customer. If the customer or prospect has another 45 minutes of questions, that's fine. They are in control and driving the conversation. Remember, you've created only 18 minutes of pure content. 

Keep it tight to keep your audience engaged.