How much sleep did you get this weekend??
Sleep - is there any health problem that isn't linked to it? In recent years, research has blamed a lack of shut-eye to everything from high blood pressure to cancer and obesity.
But it's not just our physical health that suffers. Here, expert Dr Chris Idzikowski reveals the latest thinking about sleep . . .
A GOOD NIGHT'S SLEEP IS VITAL FOR MEMORY
We tend to see sleep as the body's opportunity for stopping, but in truth it is one of the most active periods of the day (or, rather, night) for the brain, and it's vital for memory, learning and brain function.
A complete adult sleep cycle lasts 90 minutes, and we go through four or five of these cycles in a healthy night's sleep.
Each cycle is made up of distinct stages, characterised by different brainwaves.
We start with so-called 'drowsy' sleep. During this time we're crossing the threshold into slumber and we might experience dream-like hallucinations that appear at once real and fantastical.
Next, we move into light sleep, and then slow-wave or deep sleep.
Scientists have discovered that deep sleep, in particular, is when the brain consolidates declarative memories - our memory of facts and figures, events and occurrences.
During deep sleep, regions of the brain associated with memory and learning, specifically the hippocampus and the neo-cortex, communicate with one another. New information that has been temporarily stored in the hippocampus is transferred to the neo-cortex, where it becomes part of our long-term learning.
However, if you were thinking that this means you can play facts and figures into your brain as you sleep and expect to wake up with them permanently embedded, think again.
It appears that new information must have passed into the hippocampus at least an hour before you go to sleep. We complete the sleep cycle by rising again to light sleep and then entering a period of REM (rapid eye movement), or dreaming sleep.
As the period of REM draws to a close, we experience a momentary waking before beginning the next 90-minute cycle of the night. In terms of brainwaves you're awake, but your eyes likely won't open and most people don't even notice that they've risen to the surface of sleep before descending again into a new phase.
Overall, we spend up to 5 per cent of the night in drowsy sleep, up to 50 per cent in light sleep and up to 25 per cent in deep sleep. Around 20 per cent of the night is spent in REM sleep.
. . . IT ALSO HELPS YOU RIDE A BIKE
During REM or dreaming sleep, the brain shuts off the nerves that feed into the spinal column, temporarily paralysing the body to prevent us acting out our dreams.
Not everything is paralysed, though. During this stage, our eyes move beneath our eyelids, our heart rate and blood pressure increase, and men have erections, while vaginal secretions increase in women.
We don't know quite why this happens, but one theory is that even though we don't always recall them, dreams are intrinsically exciting and the body responds accordingly.
Studies have shown that REM sleep is essential for forming procedural memories (also called implicit memory or muscle memory). This is our record of how to do things using our motor skills, from doing up buttons to riding a bike or driving a car.
Over time, these skills feel automatic, so you don't even have to actively remember how to do them.
One study showed that participants who were deprived of REM sleep after learning a new skill had impaired ability to perform that new skill when they woke up.
WHAT FOOD GIVES YOU VIVID DREAMS?
Many people say that a ripe blue cheese will give you vivid dreams - but there doesn't seem to be any scientific evidence for this.
On the other hand, a study in 2002 revealed tentative findings that vitamin B6 supplements might promote more vivid dreams.
It is thought that the vitamin converts amino acids into the brain chemical serotonin, which may be involved in dreams.
You can find B6 in such foods as avocados, Marmite, broad beans, bananas, molasses, salmon and herring.
GET MORE DAYLIGHT TO BEAT INSOMNIA
Light is crucial for good sleep, so have a ritual of getting some daylight on to your face as soon as you wake up. Light receptors in the retina - the thin layer of tissue at the back of the eye - send chemical messages to your brain to confirm that the morning is here and that it's time to wake up.
This not only means you'll start to wake up, it will also set your biological clock so that you feel sleepy at the right time in the evening, too. Simply open the window and let the light flood your face for a few moments.
It's a good idea to get a decent amount of daylight throughout the day, too. A study published this month by Northwestern University in the U.S. found employees working in offices with windows slept an average of 46 minutes more per night than those in windowless workplaces. The researchers said this was due to the first group having more light exposure.
WHEN WOMEN ARE TOO HOT TO SLEEP
In general, women need around 20 minutes more sleep than men in every 24-hour period.
No one is certain why this would be, but scientists at Loughborough University have hypothesised that in order to perform multiple, complex tasks simultaneously, women use more of their brains than men do, so they have greater need for consolidation time - and that means a greater need for sleep.
The bad news is that women are more likely to be sleep-deprived than men. One Canadian study found 35 per cent of women said they have trouble sleeping, compared with only 25 per cent of men.
Women tend to be lighter sleepers, disturbed more easily during the night than men. This may be partly a result of evolution.
When you're asleep, certain brainwaves are emitted that dumb down your response to external stimuli such as noise. But the brain will let through any 'essential' noise - such as the sound of your baby crying.
Another reason women are more likely to sleep badly is down to the menstrual cycle.
When the egg has been released from the ovarian follicle and the remnants of the follicle have released the hormone progesterone, a woman's temperature rises by up to 0.4c.
This slight increase can make a woman feel too hot in bed, making it harder to get to sleep or to stay asleep.
The menstrual cycle also disrupts a woman's rhythms for releasing melatonin - the hormone that regulates your biological clock.
Interestingly, studies show that oral contraceptives reduce the amount of deep sleep women get and increase the amount of light sleep.
They also increase body temperature throughout the menstrual cycle (and even for a while after you stop taking the Pill), though it's not clear why.
WHY LACK OF SLEEP MAKES YOU HUNGRY
If you have had too little sleep, your secretions of the hormone leptin may be up to a third lower than healthy sleepers.
Leptin is the 'satiety' hormone, and helps us to feel full when our calorie intake has reached appropriate levels.
As a result, studies show you may consume roughly 900 calories a day more than you actually need in order to feel full.
COOL DOWN TO GET TO SLEEP
Your average body temperature is 37c, but over the course of the day it undergoes tiny fluctuations above and below the average of approximately 0.5c.
In a healthy adult, body temperature is at its highest around 11pm. After this peak, it begins to fall, and this is one of the triggers that we think tell the body that it's time to sleep. Body temperature reaches its lowest point at around 4am.
Research suggests that the best time for dropping into slumber is when body temperature is falling at its fastest. For this reason, sleep specialists recommend having a hot bath about 90 minutes before you try to sleep. Then, when you get out of the bath, your body temperature falls rapidly. You should also keep your bedroom relatively cool.
NO NEED FOR THE BATHROOM AT NIGHT
During sleep, nutrients continue to be absorbed from food we've eaten throughout the day, but activity in the intestines slows down.
Most importantly, the peristaltic waves that usually carry waste into the anal canal keep waste back, minimising the need to get up. This is why we often need to go to the toilet first thing, when the system reverses again and the night's waste is pushed on.
Increasing age can cause you to need the toilet more often during the night.
In men, this can be a sign of benign prostate enlargement or, more rarely, prostate cancer.
If you have noticed you are waking up several times a night to go to the bathroom, see your GP.