HC 562 The effect on energy usage of extending BST

Memorandum submitted by Dr Michael B. Morrison (BST 05)

Time for more light? Useful Daylight versus Inconvenient Darkness

Executive Summary

This short paper has been prepared to investigate the effects of different clock arrangements on available daylight in the UK using sunrise and sunset data for northerly and southerly locations. I introduce the concepts of Useful Daylight (daylight when you need it) and Inconvenient Darkness (darkness when you wish it were light). Four clock patterns are compared: GMT (Greenwich Mean Time), GMT/BST (British Summer Time), GMT/DST (double summer time) and CET (Central European Time). Of the options considered, Double Summer Time offers the greatest benefits and the least inconvenience.


1. It’s that time of year again. The sun shines brightly, yet the air is cooler. Most apparent of all, as they say in Scotland, ‘the nights are fair drawin’ in’! Summer and more specifically British Summer Time (BST), officially ends at 1:00 a.m. Greenwich Mean Time (GMT) on the last Sunday in October when the clocks are moved back one hour once again.

2. We are not alone. The entire European Union undertakes the clock change at exactly the same time. This has been the case since 1996 (the 9th EC Directive [1] passed in January 2001 prescribed indefinitely the start and end dates of summer time as the last Sunday in March and the last Sunday in October respectively). Where the UK (along with the two other westerly EU member states, Ireland and Portugal) differs is that it remains one hour ahead of the bulk of the EU population which is on Central European Time (CET) throughout the year [2] .

3. Time Zones [3] provide two principle benefits. Firstly, they are a way of organising time so that people living in a large area (a country or part of a continent) share the same clock settings. Secondly, they provide a means to organise economic and social activity around the typical hours of daylight for that region. Within this geographic framework of time zones, countries may nevertheless choose to set a different time from the zone in which they reside for political, economic and/ or social reasons. For example, Spain has adopted CET even though Madrid lies 4 degrees to the west of London.

4. Within a given Time Zone, in many regions around the world (typically those that are distant from the equator and hence have large seasonal variations in daylight), the clocks are moved forward in summer months to further enhance the alignment of economic and social activity with the daylight hours. Hence we have the British Summer Time arrangement of today. But is BST the best we could do? This short paper investigates whether greater economic benefits could be derived from adjusting the clocks to make even better use of daylight hours. For example, the clocks could be moved forward 2 hours in the summertime (Double Summer Time). Or, the UK could converge with the bulk of the EU population and move from GMT to CET (i.e. with the same summer time adjustments as at present).

5. Such changes would have a multi-facetted impact on life in the UK. There would be important effects in:

· Energy use and costs (especially lighting)

· Environmental effects (CO2 emissions);

· Road safety (road deaths and injuries);

· Leisure time and activities (physical health);

· Individual wellbeing (mental health);

· Crime (personal and family safety);

· Tourism (within the UK).

6. CET clock time convergence might also bring additional economic benefits to UK business. The 1-hour time difference may have a detrimental impact on trade and its removal could lead to improved business effectiveness, productivity and UK competitiveness.

7. Such proposals to change the clocks are never without controversy. Indeed, all attempts in recent times to introduce such changes have failed in Parliament. The crux is darker winter mornings and their perceived impact on people who rise early for work (e.g. farmers) and people in more northern latitudes (northern England, Northern Ireland and Scotland). Successful lobbying of ‘northern’ MPs has ensured that political debate has focused on the divisiveness and inequity of changing the clocks. As a consequence, past Governments have shied away from such a potentially contentious policy change. These issues rightly need to be addressed, but also re-examined in the light of important ongoing changes in UK society, politics, the economy and the environment.

8. All of these effects, both potential benefits and costs, need to be considered carefully and weighed up in order to assess whether there are compelling reasons to contemplate such a major change to British life. In this paper, we first look at the fundamental data to understand how much daylight that we receive in the UK. Then, we consider, how much of this available light that we actually make use of. Finally, we consider different clock change arrangements and their impact on useful daylight (as well as inconvenient darkness).

How much daylight do we get?

9. The Earth rotates about its polar axis (24 hours per day) as it orbits around the Sun (365 days per year). The Earth’s rotation results in diurnal cycles of daylight and darkness with the Sun appearing to rise in the east and set in the west at any given point on the surface. Proximate to the Equator, there are 12 hours of daylight each day throughout the year. However, because the Earth’s polar axis is tilted (23.5 degrees) to the plane at which the Earth orbits the Sun, all other points on the planet’s surface experience significant variation in the amount of daylight received. The number of hours of daylight and darkness within each day varies depending upon latitude (the tilt effect) and the season (the Earth’s location in its elliptical path around the sun). At the poles, the Sun is either up for the full 24 hours (summer) or down for the full 24 hours (winter) for around 6 months each year.

10. In between these extremes, the hours of daylight follow a seasonal pattern through the year. On 21 March, the Vernal Equinox, the sun rises exactly in the east travels through the sky for 12 hours and sets exactly in the west everywhere on Earth(on the Greenwich Meridian, 0 longitude, the sun rises at 6:00 am GMT and sets at 6:00 pm GMT). After the Spring Equinox, the sun increasingly follows a higher path in the sky until 21 June, Summer Solstice – the longest day. After the Summer Solstice, the sun follows a lower and lower path until it reaches the point where it is in the sky again for exactly 12 hours. This is the Autumnal Equinox which occurs on 21 September. After this the sun continues to follow a lower path each day until it reaches its lowest path on 21 December, Winter Solstice. The path then lengthens until Vernal Equinox is reached, once again giving exactly 12 hours of daylight.

11. The amount of daylight experienced between the two Equinoxes is dependent on the latitude at which you are located. The further you are from the Equator (i.e. in the northern hemisphere, the further north you are) the less daylight hours per day experienced in the winter period and the more daylight hours per day in the summer period.

12. The bulk of the UK lies in latitude between 50 degrees and 60 degrees north of the Equator. Whether you live in Lerwick in the Shetlands or Truro in Cornwall (nearly 10 degrees of latitude apart) you experience on average about 12.5 hours of daylight per day over the year [4] . However, the amount of daylight varies significantly through the seasons and between the south and the north of the country. In December, Lerwick only averages 5.9 hours of daylight per day compared with 8.1 hours in Truro. On the other hand, in June it hardly gets dark in Lerwick which experiences 18.9 hours of daylight while Truro gets 16.4 hours.

13. The sun obviously rises earlier the further east your location. However, the UK, being a relatively narrow country, fits comfortably into one time zone – Norwich 1.3 degrees east of Greenwich, sees sunrise about 35 minutes before Londonderry, 7.3 degrees to the west of Greenwich. About 95% of the UK population lies within a 5 degree zone of longitude, equivalent to 20 minutes difference in sunrise time.

14. On the other hand sunrise and sunset times vary significantly from north to south through the year. In Aberdeen in late-December the sun rises at around 08:45 GMT and sets around 15:30 GMT, whereas in mid-June the sun rises at 03:15 GMT (i.e. 4:15 BST) and sets at 21:10 (i.e. 22:10 BST). In London in December the sun is up about 45 minutes earlier than in Aberdeen, at 08:00 GMT and sets about 30 minutes later at 16:00 GMT giving about 1 hour and 15 minutes more daylight. In contrast, in mid-June the sun rises about 30 minutes later in London (03:45 GMT) than in Aberdeen and sets at 20:20 GMT, 50 minutes earlier, giving London about 1 hour 20 minutes less daylight.

15. In total, Aberdeen receives 4,501 hours of daylight per year (51% of the total number of hours in the year), while London receives 4,477 hours, 24 hours less daylight due to its lower latitude.

How much daylight do we use?

16. Clock changes have no impact on the number of hours of daylight and darkness at any given location. It is our lifestyles and the constraints within which we live our lives that determine how we make use of the available daylight or choose to carry out activities when it is dark.

17. Most people prefer to be active when it is light and to sleep when it is dark. We value daylight for a range of reasons – it reduces costs (less energy use for lighting), it increases safety (less accidents on the roads), it increases scope for outdoor leisure pursuits, and it has a positive psychological effect.

18. We would also expect the value of daylight to vary over the course of the day. Prior to waking from our slumbers it presumably has zero value [5] . On weekdays, its value increases as the population heads off to work, school, etc. then probably falls somewhat during the period while most people are obliged to work indoors. Its value will again pick up as people depart their place of work and with the commencement of the evening’s leisure hours. Later in the evening, the value of daylight will decline again as people tire and ultimately retire to bed. On weekends, the value of daylight would presumably vary less through the day and may even have a higher value than on weekdays given the greater time available for leisure. It is for these reasons that it is of national interest to consider the compatibility of our daylight hours with the lifestyles that we lead (as well as the constraints that are imposed upon our lifestyles) in the UK.

19. A significant research effort would be required to investigate and assess the value of daylight in the UK. Alternatively, a relatively simple model of "Useful Daylight" could provide important insights into alternative clock change arrangements. For the purposes of such analysis, I have adopted a definition of Useful Daylight for the bulk of the population as between 08:00 (or sunrise if it is after 08:00) and 22:00 (or sunset if it is earlier than 22:00). Based on a calendar year of sunset and sunrise times for London and Aberdeen, I have calculated the amount of useful daylight that each city receives under different clock change arrangements. It is also possible to determine the number of morning hours of "Inconvenient Darkness" (i.e. on those winter days when the sun rises after 8:00). The results of this analysis are reported in the following section.

Clock changes, useful daylight and inconvenient darkness


20. First, let’s consider the situation with no clock change arrangements in place, i.e. the time in the UK would be GMT all year round. In London and Aberdeen, as noted earlier, there would be 4477 hours and 4501 hours of daylight per year respectively. However, when we impose our Useful Daylight definition, London would have 3693 hours of useful daylight while Aberdeen would have 3719 hours, both amounting to around 83% of the available daylight.

21. Under GMT, London would only experience a total of 1.6 hours of Inconvenient Darkness in the mornings over the year, i.e. during those winter days when the sun rises after 8:00. Aberdeen, on the other hand, experiences 40 hours of inconvenient darkness in the mid-winter months.


22. Now let’s turn to the current arrangements in the UK, where the clocks are set to GMT during the winter months and to GMT plus 1 hour (i.e. BST) in the summer months. Obviously there is the same number of total hours of daylight. However, the number of useful hours increases from 3693 in London to 3903, an extra 210 hours of useful daylight. Aberdeen also benefits, with useful daylight increasing from 3719 hours to 3926 hours, an extra 207 hours. In both cases, this approximates to an average of 1 extra hour of daylight per day during the summer period.

23. Since the clocks are unchanged in the winter months from the previous example, there is no change in the number of hours of inconvenient darkness.


24. In the next example, we continue to maintain the clocks at GMT in the winter months, but put the clocks forward by two hours in the summer months. This is Double Summer Time, or in effect the UK would move onto CET during the summer months.

25. Under this arrangement, London would experience 4099 hours of useful daylight, i.e. a further 196 hours of useful daylight over and above BST and making use of 92% of the total available hours of daylight in the year. Aberdeen would experience 4067 hours of useful daylight, an increase over BST of 141 hours and accounting for 90% of the total available hours of daylight. Aberdeen benefits less than London because during the mid summer months, the sun sets after 22:00 for a longer period (i.e. deeming the light that is available ‘not useful’ under our assumption).

26. Again, since the clocks remain at GMT during the winter months, there is no change in the number of hours of inconvenient darkness.


27. In the final example, we move the clocks forward by 1 hour throughout the year so that the UK moves onto Central European Time.

28. This arrangement results in London receiving 4180 hours of useful daylight, 277 hours more than under the current GMT/ BST arrangement (93% of total available daylight) and 81 hours more than under the GMT/ DST arrangement. These extra 81 hours of useful daylight arise in the winter months since the clocks are identical during the summer months. Aberdeen receives 4104 hours of useful daylight under CET, 178 hours more than under the existing GMT/ BST arrangements but only 37 hours more than under GMT/ DST.

29. However, under CET there are also big changes in the number of hours of inconvenient darkness. Whereas in the previous examples, London experienced virtual no inconvenient darkness (1.6 hours in total for the year), under CET there would be 75 hours of inconvenient darkness on winter mornings (i.e. an extra 73 hours). This almost offsets the extra 81 hours of useful daylight obtained over and above the GMT / DST arrangement. Aberdeen, on the other hand, experiences an increase from 40 hours of inconvenient darkness to 158 hours, an extra 118 hours compared with only 37 hours of extra useful light under the GMT/ DST arrangement.


30. The benefits of BST are clear – over 200 hours of extra useful daylight throughout the UK compared with remaining on GMT throughout the year. Also importantly, inconvenient morning darkness is unchanged between the two arrangements, with there being almost none in the south increasing to around 40 hours in northern latitudes. The benefits of DST are even greater throughout the country with the south experiencing nearly a further 200 hours of useful daylight and the north an extra 140 hours resulting in the country benefitting from over 90% of the total hours of daylight available. DST would seem to be unequivocally better than BST – significant extra useful daylight and no increase in inconvenient darkness. CET is more debatable. About 80 further hours of useful light are obtained in the south compared with the GMT/ DST arrangement but this is offset with around 70 extra hours of inconvenient darkness. The analysis provides a clear explanation for the antipathy towards a switch to CET in northern latitudes of the UK with only 40 extra hours of useful daylight (compared with GMT/ DST) but with around 120 extra hours of inconvenient darkness on mid-winter mornings.


Daily sunrise and sunset data used in this paper were obtained from the website www.adventist.org . I acknowledge the Seventh-day Adventist Church for providing this service.

September 2010

[1] Directive 2000/84/EC of the European Parliament and of the Council of 19 January 2001 on summer-time arrangements.

[2] Finland , Latvia , Estonia , Lithuania , Greece and Cyprus are on Eastern European Time (EET) which is one hour behind CET and two hours behind the UK , Ireland and Portugal .

[3] There are 24 World Time Zones from -12 through 0 (GMT) to +12. Each one is 15° of Longitude as measured East and West from the Prime Meridian of the World at Greenwich , England .

[4] In fact, Truro averages about 12.3 hours/ day while Lerwick averages 12.5 hours/ day. The Scots actually receive about one day’s worth of daylight more than much of the English population.

[5] It may even take a negative value if you are woken up by the early morning light before you wish to get up in the summer months!