IT'S ALMOST MIDNIGHT in Hong Kong, a time when, according to the rules of nature, everyone should be safely tucked up in bed. But look around and people are behaving as if the day had only just begun, shopping for groceries, working on their computers or even at the gym.
In our 24-hour society, night time is no longer reserved for sleep. It's become the new daytime, offering us the chance to catch up on everything we didn't manage to cram in during the day.
Such flexibility comes at a high price, however, because many Hongkongers simply aren't getting enough sleep. 'People are trying to compress their sleep because they want to spend more time working, having fun or going out,' says Dr Wing Yun-kwok, associate professor of psychiatry at the Chinese University, who is head of the Sleep Assessment Unit at Sha Tin Hospital.
And it's not just our 24/7 lifestyle that's robbing us of shut-eye. 'The high population density in Hong Kong increases the level of brightness at night, and the houses are so tightly packed together that you're very lucky if you're not disturbed by your neighbours,' says Professor Pang Shiu-fun, vice-president and chief technology officer at CK Life Sciences International (Holdings) and an expert on the sleep hormone melatonin.
A recent Chinese University study found that about 20 per cent of the population feels constantly sleep deprived, with 20- to 30-year-olds the worst affected. And last year, a study by the University of Hong Kong found that 92 per cent of Hongkongers have experienced one or more sleep deprivation symptoms, including sleeping late at weekends, needing an alarm clock to wake up, falling asleep while watching TV and, more worryingly, feeling drowsy while driving.
Sleep deprivation is an increasingly common problem throughout the commercialised world. Researchers say we're sleeping less than a century ago.
Technology has allowed us to disregard the natural cycles of night and day, but our bodies are still slaves to the same internal rhythms that kept our prehistoric ancestors ticking.
Each of us is governed by a body clock, which is a gland called the suprachiasmatic nucleus in the hypothalamus at the base of the brain.
This controls what are known as the body's circadian rhythms, keeping every cell and tissue working under a tight regime. Through rhythmic surges of hormones and cycling in body temperature, it dictates the times we sleep, eat and play. In the morning, as daylight enters the retina, our levels of melatonin drop as our levels of the stress hormone cortisol rise.
Our rhythm is naturally set to bring on sleep twice a day: at night when melatonin is increased in preparation for sleep and there is a natural 'trough' of circadian rhythms, and in the afternoon, when there is a smaller trough.
If these cues are ignored, the sleep/wake cycle falls out of sync with hormone secretion, and the whole body functions below par, says associate professor of psychiatry Dr Chung Ka-fai, who runs Queen Mary Hospital's Sleep Disorders Clinic.
There's still some debate about the purpose of sleep. 'For most, the sleep/wake cycle co-ordinates with the light-darkness cycle, so one explanation is that sleep has a survival function. For our ancestors, going out at night would mean they would be more likely to get hurt,' says Chung. 'The other explanation is it allows the body to restore its reserves and repair itself. More recent research has also focused on memory consolidation.'
There is, however, no question about the detrimental effects of running up a sleep debt. 'Your chances of developing depression are higher, and headaches, muscle fatigue and stomach upsets can result,' says Wing.
Long-term deprivation can affect your immune system and hormonal balance, says Chung. 'In the short term, you're more irritable, your mood is worse, and your daytime cognitive functions are impaired so that you think more slowly, you're less creative, and you're more likely to make errors.'
Experts say individual needs vary from six to 10 hours' sleep per day. Nobody can alter their true requirement, and people will always revert to form if allowed to sleep freely.
According to the Chinese University study, the mean length of sleep in Hong Kong is 7.08 hours. On average, Hongkongers go to bed at 11.42pm and wake at 7.24am.
Sleep deprivation is often a symptom of a highly competitive working environment. In a survey of workers from 70 countries by Swiss bank UBS last year, Hong Kong employees were found to be working the longest hours. The study calculated employees were clocking up an average of 46 hours a week.
Much of the damage from disrupting the biological clock has been seen in shift workers. The fatigue and disorientation following night shifts is similar to jet lag, and although the body adjusts over time, the wake/sleep cycle is never totally reversed.
'People who work through the night tend to feel more fatigued and have attention problems,' says Wing. And once incurred, sleep debts are not easily repaid. 'You can't compensate by lying in at the weekend.'
But you can minimise the damage by improving sleep quality. Employers can improve the situation. For example, shifts should be kept as constant as possible. 'One month of overnights followed by three months of day shifts is better than alternating from week to week,' says Chung. When changing shifts, the new regime should be phased in slowly, going forwards rather than backwards in time. 'It is much easier to do a forward shift than a backward one.'
As Hong Kong becomes increasingly 24-hour, it could become a city full of walking disaster areas.
'In modern society, you can no longer afford to have everybody asleep at midnight,' says Wing.
'But with sleep deprivation, there are more traffic accidents, industrial accidents and domestic accidents. This is a serious public health issue in Hong Kong that needs to be addressed.'