Many of us experience an increased need for sleep when we're sick, which makes sense; avoiding over-exertion while the body fights off infection or illness seems logical. Researchers from the University of Pennsylvania found a single gene behind the urge to sleep while sick in fruit flies. They published their findings in Science, and it's a big deal; the gene itself, which they called nemuri (the Japanese word for sleep), is one of the most tangible bits of concrete proof that sleep and the immune system are biologically connected.

When the nemuri gene was removed from the fruit flies, they stopped feeling the need to sleep when ill — but when it acted normally, the fruit flies slept, and survived being sick in great numbers. If the same thing applies in humans, nemuri is what makes us sleep when we're sick — and keeps us alive while our immune system fights off threats.

Sleep apnea, the condition where breathing stops temporarily during sleep, is pretty common; the National Sleep Association has estimated that around 18 million American adults experience it. A new study revealed that having sleep apnea has an unexpected affect on the brain: it impacts autobiographical memory. The research, published in the Journal of the International Neuropsychological Society, focused on obstructive sleep apnea (OSA), the most common type of the condition, where breathing stops because airways become too narrow for air to pass through. And researchers found that people with OSA had difficulty recalling personal memories.

The people with OSA over-generalized their childhood memories, and while they remembered incidents and important events, were much less likely than people without OSA to remember details, like names or dates. The worse their memories, the more likely they were to have symptoms of depression. Good autobiographical memory is important for our mental health, and it seems that sleep apnea, and its impact on long-term memories, can seriously compromise mood.

The idea of learning in your sleep has been around for ages, but new research in January showed that it's more than a fable. It won't exactly mean you wake up fluent, though. The study, by scientists at the University of Bern, found that learning new vocab in your sleep can depend on when it's played to you.

In a deep sleep state, brain cells go through slow waves; they're active for a brief time, then lull into inactivity. The Bern scientists played pairs of words — one actual German word, one nonsense word — to German-speaking people at various stages of sleep. If the pairs were repeated numerous times during an "up" state, when the brain was active, the people were more likely to match the words into pairs when they were awake. It's a sign that the brain can sort and try and understand new words even while we're asleep. Don't rely on it instead of your Duolingo, though.

According to research published in Nature in January, sleeping people aren't always as dead to the world as they appear. It turns out that, when we're asleep, we still listen to the world around us with a small portion of our brains, keeping track of noises that might be relevant. "Sleepers enter a ‘standby mode’ in which they continue tracking relevant signals, finely balancing the need to stay inward for memory consolidation with the ability to rapidly awake when necessary," explained the researchers.

They tested this by looking at the electrical brain signals of sleeping people while surrounded by various kinds of noises. When they weren't in deep sleep, their brains 'spiked' whenever they heard noises that carried meaning, like familiar voices, while ignoring other noises. When they entered properly deep, non-dreaming sleep, though, the effects vanished. If you're just dropping off to sleep or waking up up and think you're 'hearing' what's happening around you, you're not wrong; your brain is still listening, to make sure you can wake up and deal with a threat if necessary.

REM sleep is the period of sleep where we dream, and a January study in PNAS reveals that stress directly impacts how much REM sleep we have. According to the research in mice, mild daily stress increases the mice's amount of REM sleep before it impacts any other sleep stage. Abnormalities in REM sleep are closely tied to depression and mood disorders, so stress might be affecting mood via our sleep. That wasn't the end of the story, though.

The researchers also found that when stress impacted REM sleep, the brain also physically changed its structure, particularly in the hippocampus. Various cells died, while others were grown — and that's important, because the hippocampus is a big part of our response to stress and how we deal with it. It seems that in mice, increased REM sleep in response to stress also makes the brain change its shape, to deal with the stress it might encounter while it's awake.

New science is constantly appearing about the impacts of sleep deprivation, and two new studies in January added to that picture. One, from the Washington School of Medicine, found that sleep deprivation in both mice and people increases the amount of a particular protein, tau, which is found in clumps in the brain of people with Alzheimer's. The less sleep you get, the more your brain accumulates this protein that can make it vulnerable to long-term issues like Alzheimer's.

The other study, from the American College of Cardiology, focussed on another area of human health: the heart. They tracked the sleep of over 3,000 Spanish adults and found that those who slept under six hours a night regularly were up to 27 percent more likely to have plaque build-up inside their arteries than those who got an average of eight or nine hours. Plaque build-up narrows the arteries, and is associated with cardiovascular issues and heart attacks.

What happens in your brain when you sleep? Quite a lot, as it turns out. A study published in Nature found that when we sleep, our neurons fire in many different ways, differing according to our sleep stage and where in the brain they're located. During REM sleep, the hippocampus neurons don't fire very much, while those in the frontal cortex show a lot of activity. In non-REM sleep, meanwhile, neural firing – the amount of electrical activity — evened out in both areas, and the communication between neurons slowed down.

What does this mean? Well, understanding brain activity during sleep is a big deal, because our brains do a lot of work while we snooze. Sleep is crucial for the consolidation of our memories, cleans toxic substances out of our brains, and helps cells refresh themselves for the day ahead. It turns out that the neural cells in our brains are doing more complicated work than we thought — and that could help us understand how mechanisms like memory and learning during sleep actually work.

Enjoy waking up at the crack of dawn? You're an early bird — and research indicates that the tendency to wake and sleep early is largely determined by genetics. Research published in January gave us more evidence for that fact, by expanding the amount of genetic loci, or genes in specific locations on chromosomes, associated with early-bird tendencies. And they expanded it a lot — from 24 genetic loci to a whopping 351.

The research, published in Nature Communications, found that these 351 distinct genetic loci are all associated with circadian rhythms that naturally tend towards early-to-bed-early-to-rise sleeping. It was a very big study, using data from 85,760 people, and found that these genetic signals make a significant difference. The subjects with the most early-rise genetic loci woke up 25 minutes earlier, on average, than the people with the fewest loci. If you want to change your early-bird tendencies, this research shows, it might be harder than you think; it could be encoded in your genes.

Sleep is more complicated than it appears. As 2019 goes on, we'll probably experience more wild and weird revelations about how it works — and may end the year with our minds totally blown. One thing's universally true, though: sleep is great and very useful for our health. Get more if you can.