I almost forgot to mention this poem, by Robert Louis Stevenson, from <em>A Child's Garden of Verses,</em> published in 1885 (before the institution of British Summer Time in England). 

Bed in Summer

In winter I get up at night
And dress by yellow candle-light.

In summer quite the other way,
I have to go to bed by day.

I have to go to bed and see
The birds still hopping on the tree,

Or hear the grown-up people's feet
Still going past me in the street.

And does it not seem hard to you,
When all the sky is clear and blue,

And I should like so much to play,
To have to go to bed by day?

Well, for starters, it only makes sense in latitudes that are far enough from the Equator to have a big change in day length between summer and winter.

We have had it in most of the United States for as long as I've been alive. Mostly people like it and the evidence is that it does save a <em>small</em> amount of energy. Generally farmers hate it, though. It would probably save more energy if our buildings were designed to make better use of available daylight--in the fifties when energy was cheap nobody cared about that, and old designs like the clerestory factory "sawtooth" roofs were abandoned.

An interesting question is <em>why it is necessary to change it suddenly, all at once, in a one-hour step</em>? Most clocks now are controlled either from the powerline frequency or, more and more, by time-setting signals over the Internet, so why not have a continuous, smooth adjustment of a fraction of a minute every day? It would drive people with expensive Rolex watches crazy, of course.

Probably it doesn't work.

Conscientious people do works.

It can improve to some measure yet it cannot get so much as most people can expect. Daylight hours is helpful I have to admit. There is an alternative that can help more which is work at home by way of internet collaboration.  Besides, electricity-driven plane is also a very promising new technique to save more energy in the future generation.

It is an inspiring theme.

Thank you Andrew for your view .This is an interesting topic for me and I know that this information can be found online, but I asked this question here because not everybody think the same and maybe that's why there are many countries that have not adopted this measure and others who did a time and then they left and it's interesting to know what people from different countries think about this topic and at the same time I'm  practising my reading and writing in a funy way :)