The combination of sunshine and England's progress in the World Cup means the country is in a very optimistic mood, says Dr Cliff Arnall, of the University of Cardiff.

He has devised a formula for measuring mood through outdoor activity, energy levels and sunlight. He used a similar method to calculate that Jan 23 was the most depressing day.

His formula is O + (N x S) + Cpm/T + He, in which O stands for being outdoors and outdoor activity, N for nature, S for social interaction, Cpm for childhood summers and positive memories, T for temperature and He for holidays and anticipating time off.

Today was pinpointed as the happiest day of the year because of its added "thank goodness it's Friday" feeling at the start of a summer weekend.

Dr Arnall said: "I wanted my formula to prove the key to happiness can really be simple."