Sleep deprivation is awful, and for many, it’s simply an accepted part of daily life. No matter how much research emerges – the type that points out that a lack of shut-eye increases our risk of cancer, heart attacks, Alzheimer’s, stroke, and kidney disease – most of us are still not getting enough.

A new Nature Medicine study has now revealed why you feel so spaced out after a lack of sleep: your neurons fire far more slowly, which means that your short-term memory recall is inhibited. Your brain is essentially running in slow-motion.

Led by the University of California Los Angeles (UCLA) and Tel Aviv University, this team of researchers is arguably the first in the world to find direct causal links between sleep deprivation and hampered cognitive reasoning – a mechanism which has surprisingly never been fully explained.

Examining such neurological links is always fairly difficult. To do so, you need to watch how individual neurons behave; an external scan, one which tracks the general propagation of electrical activity in the brain, isn’t precise enough.

For this study then, the team asked 12 people who were preparing to undergo surgery for epilepsy to help. They already had electrodes implanted in their brains as part of their ongoing treatment, which were primarily designed to pinpoint the origin of their seizures.

In order to speed up the onset of a seizure for research purposes, the subjects were asked to not sleep. Post-sleep deprivation, the 12 were asked to categorize a series of images as rapidly as possible.

This test is known as a “psychomotor vigilance task”, something which aims to assess a person’s reaction speed and accuracy. It’s known that a lack of sleep dampens both, but specifically why it does has remained unclear for some time.

The electrodes, however, managed to track the activity of around 1,500 individual neurons across these 12 patients as they engaged in the task. Special attention was paid to those in the temporal lobe, the part that deals with memory recall and visual perception.

“The neurons responded slowly and fired more weakly, and their transmissions dragged on longer than usual,” lead researcher Dr Itzhak Fried, a professor of neurosurgery at the David Geffen School of Medicine at UCLA, said in a statement.

This meant that visual information couldn’t be efficiently converted into thoughts and actions – and the sleepier the patients were, the worse their neurons performed. It literally took their brains longer to register and process the world around them.