A study into aircraft noise pollution around Zurich airport shows that people are more disturbed by early morning flights than evening flights.
The report by the Federal Institute of Technology in Zurich also suggests that the people surveyed are less tolerant of aeroplane noise than before, even in areas where levels have not increased.
Switzerland was forced to open a controversial southern approach to Zurich airport after Germany banned flights between 9pm and 7am over the northern approach to the hub - which is over parts of southern Germany - in April 2003.
Campaigners were quick to take to the streets in protest despite Zurich being the only airfield in Europe to shut down its runways between 11pm and 6am.
In 2001 and 2003 some 3,000 people in 57 towns and villages around the airport were asked the by Federal Institute in two separate questionnaires how aircraft noise affected them. Their responses were converted into "annoyance" levels on a scale of 0-10.
The results ranged from 2.5 for people not under a flight path to over 7 for those in areas where noise levels have increased.
In addition, 64 volunteers underwent an experiment to see how their sleep patterns were affected by noise. Each volunteer was subjected to bursts of pre-recorded car and aircraft noise over loudspeakers as they slept.
Their heart rate and respiratory patterns were checked by measuring equipment attached to their mattresses and bedposts.
The experiment showed that people are far more likely to have their sleep disturbed early in the morning just before they wake up than late at night just after they go to bed.
Mark Brink, one of the study's authors, told swissinfo that the only way to reduce annoyance to residents would be to ban flights until later in the morning, rather than cutting the number of aircraft taking off and landing.
"A change in noise in the morning creates more annoyance than the same change in noise in the evenings," he said.
"Redistributing the morning flights would not decrease the amount of disturbance as our studies showed that the first noise disturbance is critical. People have the strongest reaction to this noise and the reaction decreases with each subsequent flight.
"The only thing that could help is to change the first flight to later."
The report, entitled Noise Study 2000, also found - unsurprisingly - that people under the new flight paths were the most annoyed.
But the study also concluded that people living in areas where noise had not increased were also less tolerant than a previous study in 1991.
"There are other factors more important than the noise itself for people's emotional reactions to this subject," said Georg Thormann, who also worked on the study.
"There is a political aspect as well. Since flights were banned over Germany and rerouted over Switzerland, people have become more sensitive."
The SFr430,000 ($330,000) study was partly funded by Zurich airport operators Unique, the Swiss transport ministry and the Federal Health Office.
None of the three was willing to give a detailed response of the findings until they had time to study the report.
swissinfo, Matthew Allen
In April 2003 flights over southern Germany into Zurich airport were banned between 9pm and 7am.
As a result, a new southern approach in Switzerland was introduced in October 2003.
Zurich airport closes its runways from 11pm to 6am.