It had been an early start on Monday for the President, who woke up in Rome at Villa Taverna, the US Ambassador's residence.
He then flew on Air Force One to Edinburgh, touching down at 10.54am, before being driven to Glasgow, arriving at Cop26 just after midday, where he was welcomed by Boris Johnson.
It is not the first time that Biden has been accused of falling asleep during important meetings.
Benjamin Netanyahu, Israel's former prime minister, suggested in a Facebook video that Biden had fallen asleep when sitting next to Naftali Bennett, the new Israeli leader, at the White House in August.
However, the video of the interaction that went viral was clipped on social media and did not show the full picture. Although Biden's head was bowed, his hands kept moving and he replied to Bennett straight away.
Questions have been raised about Biden's physical, and mental fitness, after a series of gaffes, including falling three times while climbing the stairs of Air Force One and referring to his wife Jill as his sister.
However, Biden, who has said he uses a Peloton bike to keep fit, often makes a point of jogging in a show of his fitness.
Biden's latest medical report showed no signs of any degenerative disease.
