Tiredness after an away game in European competition is an age-old complaint in football but it was undeniably a factor in Chelsea's shock 1-0 loss at home to West Ham on Saturday.
The visitors had arrived at Stamford Bridge having won just three of their opening 13 games of the season but they were energetic and physical and Frank Lampard's clearly exhausted side couldn't match Manuel Pellegrini's men for effort, after a mentally and physically draining draw in Valencia in midweek.
Consequently, Chelsea slipped to a dismal and deserved defeat, with Aaron Cresswell scoring the only goal in the game in the 48th minute, after cutting inside Reece James before unleashing a fine strike past Kepa with his weaker right foot.
It was the decisive moment of class that the under-pressure Pellegrini needed but not what Lampard would have expected at all, given Chelsea were going up a side that had lost their last three league games.
What will have been particularly disappointing for the Blues boss is that his side rarely troubled West Ham's third-choice goalkeeper David Martin, who was handed his Premier League debut in response to Roberto's awful run of form while standing in for the injured Lukasz Fabianski.
A clean sheet on his first top-flight appearance, and at Stamford Bridge, is certainly something special but the cold, hard truth is that a very ordinary Chelsea didn't put him under much pressure.
Olivier Giroud and Pedro were particularly culpable in that regard, looking every bit like players who hadn't started for the Blues since August and September, respectively.
They looked every bit as rusty as you would have expected in the circumstances and failed to impact the game before being substituted.
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Christian Pulisic, by contrast, was by far and away Chelsea's most consistent threat but it must be said that he missed the few big chances that fell to his side.
Chelsea had actually started the match well but, as their midweek exertions at Mestalla took their toll, the tempo of their play gradually dropped, and their frustration began to show even before the half-time whistle blew.
Things only got worse after the interval. Indeed, the Blues never showed up for the second half, with several players who had started against Valencia, including James, Jorginho and Mateo Kovacic, clearly struggling with fatigue.
The lack of movement up-front was sorted out late on, with Pulisic given a false nine role following the introduction of Willian and Callum Hudson-Odoi but it was a case of too little, too late.
Consequently, West Ham held on quite comfortably to the lead given to them by Cresswell to claim their first win and their first clean sheet in the Premier League since 22 September, as well as their first victory at Stamford Bridge since 2002.
Ahead of kick-off, Lampard admitted that he was acutely aware of his side's finishing problems this season. His side are ranked 17th by Opta for the conversion of 'big chances', so the loss of the in-form Tammy Abraham to injury was always likely to be keenly felt against West Ham.
This was a poor performance, though, and an even worse result.
Indeed, after putting clear daylight between themselves and the likes of Tottenham in the race for a top-four finish, the gap between fourth-placed Chelsea and Jose Mourinho's resurgent Spurs is now down to six points.
Chelsea need to return to winning ways, and fast. But with two games to come next week, at home to Aston Villa and away to Everton, one fears that fatigue could become an increasingly influential factor in the Blues' performances.