Claude Gueant, Sarkozy's chief of staff, told the AFP news agency that he was "doing well and talking normally to medical staff".
A presidential aide also told AFP that Sarkozy had a problem related to his vagus nerve - which helps the body regulate its heart rate - but that doctors considered the problem "minor".
A condition known as vasovagal syncope can reduce the heart rate and blood pressure, particularly if the victim is dehydrated.
Patients can faint but normally make a complete recovery, according to medical experts.
Sarkozy, who assumed the presidency in May 2007, is a fitness enthusiast, and is often seen jogging or cycling with aides and bodyguards.
Just over three weeks ago, the Elysee Palace released the findings of Sarkozy's latest annual health check, describing the results of recent blood and heart tests as "normal". No further details were provided.