"He has been undergoing medical examinations since Tuesday but he only slept last night," Sebastien Farran said in a text message.
"The nature of the examinations is general," he told AFP on Thursday.
The 69-year-old singer was treated for bronchitis in August at a hospital in the French Caribbean island of Martinique. He had been flown there by helicopter after falling ill on nearby Saint Barts island, where he also has a residence.
The rocker has sold more than 100 million albums and played 45 major tours in a career that began in the 1960s. He is currently in the middle of another major tour.
Hallyday sent a reassuring tweet to fans on Thursday saying he would return to the studio on Monday to record a song that will be a duet with Canadian diva Celine Dion.
Farran backed this saying recording for the new album "is scheduled to resume next week."