Just being near your smartphone can be enough to reduce your brain power, even when it's switched off, according to a new study – so you might want to give yourself and your mobile some alone time in the future.

The research shows the way our smartphones have become a constant source of distraction, whether or not we're actually using them, and could lead to a better understanding of the dangers of being always connected and available. 

According to the team from the University of Texas at Austin, the study demonstrates how having phones within sight or within easy reach means some of our brainpower is inevitably used up as we try not to be distracted.

"We see a linear trend that suggests that as the smartphone becomes more noticeable, participants' available cognitive capacity decreases," says one of the researchers, Adrian Ward.

"Your conscious mind isn't thinking about your smartphone, but that process – the process of requiring yourself to not think about something – uses up some of your limited cognitive resources. It's a brain drain."

Those conclusions were reached after two experiments. In the first, 520 smartphone users were told to turn their phones to silent then either leave them in another room, place them face down on a desk, or put them in a pocket or a bag.

The volunteers were then asked to complete a series of computer tests that required serious concentration to score highly.

The participants who left their phones in another room "significantly outperformed" those with their phones on the desk, and "slightly outperformed" those with their phones in a pocket or a bag, report the researchers.

Based on a follow-up survey, however, the participants themselves didn't feel the location of their phones had any effect on their ability to concentrate on the test.

Next, 275 volunteers were asked to go through the same process, but this time they were asked in advance how much they felt they depended on their phones. Again, phones had to be left on a desk, in a pocket or bag, or in another room, but this time some participants were asked to turn their phones off as well.

Those who said they were most dependent on their phones performed worse on the tests, but only when the phone was placed on a desk or in a pocket or a bag. If the phone was in another room, phone dependency had no significant impact on the test scores.

Whether or not the phone was on or off, or placed face up or face down on a desk, didn't make any difference to overall performance either, the researchers found.

In other words, the only way to really make sure your phone isn't distracting you is to physically remove it from the room you're in. Switching it off or placing it face down just isn't enough.

You probably don't need telling that having a smartphone around can be distracting, but it's interesting that just having a phone close by – even if it's turned off or in silent mode – is enough to reduce our mental capacity for other tasks.

"It's not that participants were distracted because they were getting notifications on their phones," says Ward. "The mere presence of their smartphone was enough to reduce their cognitive capacity."

The research has been published in the Journal of the Association for Consumer Research.