In the current weather you can get sunburnt during the day and chilled at night and the same fate awaits punters if they're not careful. Tired legs and weary minds are a legacy of summer sports – look at England's one-day cricket squad, from which Paul Collingwood and James Anderson have been belatedly "rested".
Equally, players embarking on the winter season are often not up to speed.
Take today's final Classic of the Flat season, the St Leger, for which we previously advised Mastery at 20-1, based on his second place in the Great Voltigeur at York and creditable third in the Grand Prix de Paris. But some of the field at Doncaster will have had enough, and some are just seeking consolation for failing elsewhere. It's a small stakes event for these reasons.
Meanwhile, mid-week internationals often inflict jet lag, injuries and the depression of a bad result on many Premier League players, so the weekend's two big fixtures, Manchester United at Spurs and Arsenal at Manchester City, have additional uncertainties.
Arsenal are without Andrei Arshavin and Tomas Rosicky, and though Cesc Fabregas is back, the depth of City's squad could be the decisive factor, with Carlos Tevez a midweek injury victim. United might play a rusty Rio Ferdinand but England's star Aaron Lennon could have Patrice Evra on toast again. £5 win on both City (2-1, Ladbrokes) and Spurs (23-10 Coral).
*Northampton and Pittsburgh got off to winning but spluttering starts in the Premiership and NFL. But Andy Murray, tired and injured, lost.