Why reducing sleep makes you hungry

I am something of an insomniac and I know that when I don't get at least seven hours' sleep I become tired and irritable.

I've also noticed that a bad night's sleep affects my memory. The link between sleep and memory has been around for a long time and one plausible theory is that during deep sleep your brain moves short-term memories, collected that day, into long-term storage, freeing up space in your brain for more memories.

So if you don't get enough deep sleep those memories will be lost.

Whether this theory is right or not, getting a good night's sleep (rather than staying up late and cramming) is particularly important for students who are currently revising for exams.

But what really surprised me, while making the Truth about Sleep for BBC One, was discovering how much a bad night's sleep can affect blood sugar control and hunger, even in healthy volunteers.

To find out more we asked Dr Eleanor Scott, who works at the University of Leeds, to help us.

We recruited a group of healthy volunteers and, under her supervision, fitted them with activity monitors and continuous glucose monitors, so we could see what was happening to their blood sugar levels, every five minutes or so.

Then we asked our volunteers to sleep normally for two nights (so we had a baseline), have two nights where they went to bed three hours later than normal, followed by two nights where they could sleep as long as they liked.

Ten custard creams

Naturally enough, being an avid self-experimenter, I joined in. Staying awake when you really don't want to, and everyone else in your house has gone to bed, was not enjoyable.

I was also unpleasantly surprised by just how much my blood sugar levels rose on the days when I was sleep deprived, and how hungry that made me.

The same was true of my fellow volunteers. When we met to get our results from Dr Scott everyone complained about having the munchies.

As one volunteer put it, "I wanted lots of biscuits and I didn't just have one. I'd go for 10. I wrote it down on my diary - 10 custard creams"

"Is that unusual?" I asked him.

"Well that certainly unusual for breakfast!" he replied.

All of us, whether we had feasted on biscuits or managed to stick to our normal diet, saw marked increases in our blood sugar levels, to the point where some previously healthy individuals had levels you might expect to see in borderline type 2 diabetics. These problems resolved after a couple of good nights' sleep.

As Dr Scott pointed out, there is now a lot of evidence from big studies which suggests that people who sleep for less than seven hours a night are more likely to become obese and also develop type 2 diabetes.

So why does this happen?

Dr Scott said: "We know that when you are sleep-deprived this alters your appetite hormones, making you more likely to feel hungry and less likely to feel full. We also know that when people are sleep-deprived they often crave sweet foods, which could explain the custard cream cravings.

"Also, if you're awake when you're not meant to be, you produce more of the stress hormone, cortisol, and that can influence your glucose level, as well, the next day"

Important for children

A recent meta-analysis, carried out by researchers at King's College London, found that sleep-deprived people consume, on average, an extra 385 kcal per day, which over time could certainly add up.

It's not just that your blood sugar levels soar and your hunger hormones go into overdrive when you are sleep-restricted.

Researchers have also found that areas of your brain associated with reward also become more active when you're tired. In other words you become more motivated to seek out food.

Getting enough sleep is particularly important, not just for adults but also for children.

In another recent study researchers took a small group of pre-school children, aged three-to-four, all regular afternoon nappers, and not only deprived them of their afternoon nap but also kept them up for about two hours past their normal bedtime.

The following day the children ate 20% more calories than usual, particularly more sugar and carbohydrates. They were then allowed to sleep as much as they wanted. The following day they still consumed 14 per cent more calories than normal.

All of which points to the importance of getting a good night's sleep.

Techniques that work - the results

A few weeks ago, we kicked off the BBC Sleep Challenge and 367 of you chose to test out options to help you sleep and report back.

This was not a proper scientific survey, because it was self-selecting, but it was revealing nonetheless.

Of those taking the Sleep Challenge, the most common complaint was waking up in the night (half), followed by difficulty falling asleep in first place (a quarter).

The most popular option was the controlled breathing technique which 146 people tried.

The results were fairly evenly spread, with around 50 people choosing to cut out alcohol; do morning exercise; take a warm bath or avoid social media at least an hour before bedtime.

The least popular option was eating two kiwi fruit before bed, which only attracted 27 people.

It was also the option that people who did it found the least effective - only a third said it helped, some said it made their sleep worse!

The other options produced surprisingly similar results, with around half of each group saying they had got benefit from doing the technique they'd chosen, while half did not.

It appears the techniques with the most science behind them were the most effective, but clearly nothing works for everyone.

So shop around and see what works for you. I now do most of them (I enjoy kiwi, just not every evening, and I prefer an evening shower to a bath).

I've also committed myself to eating more fibre, which was not on our original list because we thought the effect would be too slow to show up.

I'm not entirely sure which is the 'best' but the combination has certainly helped me get a better night's sleep.

Truth about Sleep, BBC One, 9pm Thursday 11 May