Who among us wouldn't like a bit more sleep?
Just last month, a survey by the University of Leeds found that an astonishing 25 per cent of people get less than five hours' sleep a night.
The researchers also noted a distinct mismatch between how much sleep people intended to get - or thought they needed - and what they actually got.
If you like to burn the candle at both ends, just because you're not nodding off at your desk doesn't mean you're not suffering for it. Here, we reveal the more subtle signs that you could do with an early night . . .
People who sleep for less than six hours a night are four times more likely to come down with a cold than those sleeping for seven or more hours, reported researchers at Carnegie Mellon University in the U.S. in September.
"Sleep deprivation affects the immune system in many ways," says Dr Victoria Revell, a chronobiologist at the University of Surrey. "It acts similarly to a state of stress - something known to have a negative impact on immunity."
White blood cells called lymphocytes, which recognise and help attack viruses and bacteria, don't reproduce effectively when we're sleep deprived.
And studies on people having vaccinations have also shown we don't develop as many antibodies (proteins the body makes to fight viruses) when we're sleep deprived, lowering protection against disease.
In 2006, scientists at Walter Reed Army Institute of Research in the U.S. found that we lose our sense of humour when we're tired.
In a study where people who'd been kept awake for 49½ hours were asked to compare cartoons and amusing newspaper headlines, they were found to have lost the ability to find the material funny.
Finding something funny is one of the most complex brain processes - it uses attention, memory and divergent thinking (where the brain has to imagine several scenarios or solutions at once), and then requires linking all of these thought patterns with a feeling.
All these processes are controlled by a part of the brain called the prefrontal cortex, located at the front of the brain, behind your forehead.
"This is the area of the brain most impacted by lack of sleep," says independent sleep specialist Dr Neil Stanley.
It's not just yawning that'll give tiredness away: sleep experts say a 'tired' voice tends to sound flat and monotone. This is because the muscles in the throat that govern the sound of our voice are not as well controlled when we're tired.
Professor Kevin Morgan, director of the Clinical Sleep Research Unit at Loughborough University, says: "This reflects the slowing and dulling of brain activity that occurs during sleep deprivation.
"Co-ordinating the musculature of the mouth and tongue to form speech is quite a complicated task, and we don't manage it as effectively when we're tired."
When you do formulate words, you may find yourself tripping over your tongue. Experts at Pennsylvania State University in the U.S. said they could spot when people were tired just by changes in the way they pronounced certain letters.
After depriving students of sleep, they compared their speech with people who'd rested well and found distinct differences in pronunciation - Ps became Bs, Ts sounded like Ds, and Ks more like Gs.
A common test for pain tolerance is to see how long someone can keep their hand in cold water.
In trials published in May from the Norwegian Institute of Public Health, those with insomnia were more likely to remove their hand earlier than those sleeping well.
Studies have also shown those with chronic pain say their symptoms are worse when tired.
"We're still trying to unpick why, but there are some theories," says Professor Nicole Tang, a health psychologist who specialises in sleep and pain at the University of Warwick. "Sleep loss lowers mood and, in a low mood state, people are more likely to be looking for pain and be more aware of it.
"We also know that levels of inflammatory chemicals involved in pain are raised after a poor night's sleep. Finally, one way the body controls pain is via signals sent down from the brain, which inhibit pain sensations - it's possible that, during sleep deprivation, this descending pathway is weakened for some reason.
"It's been estimated that a unit of alcohol drunk when someone is sleep deprived will have the same effect on performance as three units consumed when someone is fully rested," says Dr Stanley.
When men were asked to perform tasks under the influence of sleep deprivation and alcohol separately, then with both combined, reaction times, accuracy and speed suffered in each situation - but were significantly worse when alcohol and tiredness were combined, according to a study published in the journal Psychopharmacology.
"We know from numerous studies into driving and sleep deprivation that lack of sleep triggers the same decline in performance as alcohol," says Dr Stanley. "Adding alcohol simply adds to this impairment."
As we're less able to control our muscles when we're sleep deprived, our face droops - particularly the eyelids and the corners of the mouth - meaning others may think we look sad, say researchers at Stockholm University in Sweden.
'Controlling our muscles requires a level of alertness in the tissues that starts to decline when we are deprived of sleep,' says Professor Kevin Morgan.
What's more, the flow of blood to the tiny vessels close to the skin's surface slows when we're sleep deprived, which explains why we look pale. Because of this reduced blood flow, fluid can build up around the eyes, making them look puffy.
As for dark circles, "no one really knows why they form in healthy people but, if your sleeplessness is caused by sleep apnoea [where the walls of the throat narrow during sleep, obstructing breathing], it's likely that your blood is not adequately oxygenated during the night, which makes it look darker".
Rajiv Grover, past president of the British Association of Aesthetic Plastic Surgeons, adds:
"The thickness of the skin and muscles on the rest of the face hide this, but the skin under the eyes is very thin, and so this shows through."
When researchers at the University of Colorado Boulder in the U.S. asked people to reduce their sleep to just five hours a night and left them with easy access to snacks during the day, they gained an average of .9 kilograms in just one week.
"Sleep loss is almost the perfect storm for over-eating and weight gain," says Dr Stanley.
"The parts of the brain that control willpower (in the prefrontal cortex) are weakened, levels of the hormone ghrelin, which stimulates appetite, increase by 20 per cent and levels of a hormone called leptin, which tells us when to stop eating, fall by 18 per cent."
On top of this, research from Weill Cornell Medical College in Qatar has found that even just 30 minutes of sleep loss impacts negatively on metabolic processes in the body that determine how well the body responds to insulin, which can contribute to weight gain.
"Whether it affects thermogenesis (the speed at which we burn calories) is still under debate," says the study's author, metabolic researcher Professor Shahrad Taheri. "But being tired due to sleep loss is likely to predispose us to less activity during the day."
Poor sleep is known to affect memory function. "We're still trying to determine exactly what role sleep plays," says Dr Guy Leschziner, a neurologist at London Bridge Hospital Sleep Centre.
Experts suspect that sleep is a time when we consolidate memory, with the brain working out which of the events of the day we need to recall and which can be forgotten.
Sleep also seems to be a time to clear the brain of waste chemicals that build up during the day - failing to do this properly might be related to future memory loss.
In the short term, though, Dr Leschziner says the most noticeable effect of poor sleep on memory is the ability to retain information that you need in order to remember simple things, such as where you put your keys or parked your car.
"Quite simply, your level of vigilance falls when you are tired and you don't take in the information you need to remember these things."
Feeling sleepy after lunch is not a sign of sleep deprivation, says Dr Stanley. "That's normal, and related to the body clock and a natural fall in body temperature that occurs at this time." However, feeling like you could nod off at 11am is a sign you need more sleep.
This, says Dr Stanley, is the point in our body clock's cycle when we should feel most naturally energised - regardless of when we woke up - because of the surge of hormones and other processes in the body dictated by our body clock.
One thing that does seem to respond positively to sleep deprivation is our ability to think laterally. Researchers at Albion College, Michigan, in the U.S. asked students to solve puzzles such as this one:
"Two girls are born on the same day, month and year to the same parents, but they aren't twins . . . how?"
Those answering when tired were actually more likely to come up with the solution, possibly because their fatigued brains were more likely to make unusual connections.
(If you've had a good night's sleep and can't work out the answer to that puzzle, it's because they are two of a set of triplets.)