We spend on average 33% of our lives asleep, and our ability to enjoy a restful night's sleep have big impacts on our lives. The economic and social impact of poor sleep are well recognised, even if we can't put firm numbers on the actual costs.
One particular study that is interesting is one that was conducted in the USA. The US being a large country has several time zones. Eastern, Central, Mountain, and Pacific.
Time zones are about 1200 Km wide at 45 Degrees Latitude(2) and share the same time. The researchers (Gibson and Sharder) studied two cities in the USA that were in the same time zone. So all clocks share the same time (remember this it's important later). Amarillo, Texas and Huntsville, Alabama are both in the Central Time Zone. Amarillo is close to the western edge and Huntsville is close to the eastern edge.
Sunset in Hunstville is approximately 1 hour earlier than in Amarillo. In May sunset in Huntsville is usually around 7:30, but 1200 Km west in Amarillo sunset is closer to 8:30. The sleep/wake cycle in Humans is called the circadian rhythm, and is governed by a “Master Clock”(3) deep in the brain that receives signals about the intensity of light reaching the eye. It turns out that blue light has a big influence on the Master Clock (but that's for another day).
The later sunset means that, according to the clock, you are getting more light later in the day if you live in Amarillo than you do if you live in Huntsville. This wouldn't be a problem if local time took account of this (remember about the standardised time) and people started their days an hour later, to keep in line with their Master Clock.
Sharder & Gibson's study showed that typically people who lived in Huntsville went to bed earlier than those in Amarillo. But because of how life is socially constructed around the work day, they were getting up at about the same time. So they were getting less sleep. Sharder & Gibson estimated that the typical residents of Amarillo were getting an hour less sleep a week when compared to Huntsville residents. Being good economists Sharder & Gibson wanted to find out what impact an hour less sleep a week might have on income. They compared income from the Amarillo and Huntsville and other similar pairs of cities and found that an extra hour of sleep a week over the long term increases wages by around 4.5%. If you increased the average sleep time by 1 hour a night over the long term average wages increased by 16%.
The effects of total sleep deprivation have been well studied, but not so much for partial sleep deprivation.
Researchers in Finland(4) studied partial sleep restriction where test subjects spent either 3, 5 or 7 hours in bed daily for 7 consecutive days, or were in a control group that spent 9 hours in bed. The subject were then given a standard set of tasks to do, and scored.
As you might expect the 3 hour group had the worst scores, and there was a significant deterioration in performance after 1 night with only 3 hours sleep. There was also a straight line deterioration, more nights with only 3 hours sleep led to worse scores. If you had 5 or 7 hours sleep, the effect of partial sleep loss didn't start to show up in scores until you had had 3 nights at the low sleep level. After that scores stayed at a new lower level for the rest of the 7 days.
The interesting thing was that people who were in the 5 to 7 hours sleep group took longer to get back to the pre sleep loss scores than people had who had no sleep at all. Some studies have shown that a single 8 hour recovery sleep can reverse the effect of a night with no sleep. In the same study people who had had 7 nights at only 5 hours sleep took at least two 10 hour periods of sleep for scores to recover. Three nights of 8 hours sleep was not enough in either 5 hour or 7 hour sleep groups to bring score back to previous levels.
The thinking is that chronic low level sleep loss causes the brain to adapt to the stress, and this adaptation needs longer to be reversed than an occasional complete lack of sleep.
So it's not a big stretch to wonder if the 4.5% difference is due to reduced productivity through fatigue in the Amarillo & Huntsville comparison.
And we haven't even thought about negative health effects of chronic lack of sleep!
In our next post we'll talk about sleep hygiene, and things that can influence how well you sleep.
1) Sharder & Gibson, Dept of Economics UC San Diego, Aug 2015
2) Because the earth is round the closer you get to the equator the wider time zones become, and they get narrower as you get closer to the poles.
3) The technical name is the suprachiasmatic nucleus
4) Alhola & Polo-Kantola, Dept of Psychology, University of Turku, Finland