That changed in the wake of the Reformation and the counter-Reformation.Protestants and Catholics became accustomed to holding secret services atnight, during periods of persecution. If earlier the night had belonged toreprobates, now respectable people became accustomed to exploiting the hours ofdarkness.

This trend migrated to the social sphere too, but only forthose who could afford to live by candlelight. With the advent of streetlighting, however, socialising at night began to filter down through theclasses.

In 1667, Paris became the first city in the world to lightits streets, using wax candles in glass lamps. It was followed by Lille in thesame year and Amsterdam two years later, where a much more efficientoil-powered lamp was developed.

Roger Ekirch says this 1595 engraving by Jan Saenredam is evidenceof activity at night

London didn't join their ranks until 1684 but by the end ofthe century, more than 50 of Europe's major towns and cities were lit at night.

Night became fashionable and spending hours lying in bed wasconsidered a waste of time.

"People were becoming increasingly time-conscious andsensitive to efficiency, certainly before the 19th Century," says RogerEkirch. "But the industrial revolution intensified that attitude by leapsand bounds."

Strong evidence of this shifting attitude is contained in amedical journal from 1829 which urged parents to force their children out of apattern of first and second sleep.

"If no disease or accident there intervene, they willneed no further repose than that obtained in their first sleep, which customwill have caused to terminate by itself just at the usual hour.

"And then, if they turn upon their ear to take a secondnap, they will be taught to look upon it as an intemperance not at allredounding to their credit."

Today, most people seem to have adapted quite well to theeight-hour sleep, but Ekirch believes many sleeping problems may have roots inthe human body's natural preference for segmented sleep as well as the ubiquityof artificial light.

A small city like Leipzig in central Germany employed 100 mento tend to 700 lamps

This could be the root of a condition called sleepmaintenance insomnia, where people wake during the night and have troublegetting back to sleep, he suggests.

The condition first appears in literature at the end of the19th Century, at the same time as accounts of segmented sleep disappear.

"For most of evolution we slept a certain way,"says sleep psychologist Gregg Jacobs. "Waking up during the night is partof normal human physiology."

The idea that we must sleep in a consolidated block could bedamaging, he says, if it makes people who wake up at night anxious, as thisanxiety can itself prohibit sleeps and is likely to seep into waking life too.

Russell Foster, a professor of circadian [body clock]neuroscience at Oxford, shares this point of view.

"Many people wake up at night and panic," he says."I tell them that what they are experiencing is a throwback to thebi-modal sleep pattern."

Stages of sleep

Every 60-100 minutes we go through a cycle of four stages ofsleep

Stage 1 is a drowsy, relaxed state between being awake andsleeping - breathing slows, muscles relax, heart rate drops

Stage 2 is slightly deeper sleep - you may feel awake andthis means that, on many nights, you may be asleep and not know it

Stage 3 and Stage 4, or Deep Sleep - it is very hard to wakeup from Deep Sleep because this is when there is the lowest amount of activityin your body

After Deep Sleep, we go back to Stage 2 for a few minutes,and then enter Dream Sleep - also called REM (rapid eye movement) sleep -which, as its name suggests, is when you dream

In a full sleep cycle, a person goes through all the stagesof sleep from one to four, then back down through stages three and two, beforeentering dream sleep

Source: Gregg Jacobs

But the majority of doctors still fail to acknowledge that aconsolidated eight-hour sleep may be unnatural.

"Over 30% of the medical problems that doctors are facedwith stem directly or indirectly from sleep. But sleep has been ignored inmedical training and there are very few centres where sleep is studied,"he says.

Jacobs suggests that the waking period between sleeps, whenpeople were forced into periods of rest and relaxation, could have played animportant part in the human capacity to regulate stress naturally.

In many historic accounts, Ekirch found that people used thetime to meditate on their dreams.

"Today we spend less time doing those things," saysDr Jacobs. "It's not a coincidence that, in modern life, the number ofpeople who report anxiety, stress, depression, alcoholism and drug abuse hasgone up."

So the next time you wake up in the middle of the night,think of your pre-industrial ancestors and relax. Lying awake could be good for you.