"The president cannot suffer bright lights," George Charamba was quoted as saying by the state-run Herald newspaper.
Mr Mugabe, 93, has been seen apparently sleeping at several events, leading to speculation about his fitness.
He intends to stand in presidential elections next year.
The president is currently receiving specialised medical treatment for his eyes in Singapore.
Africa Live: More on this and other stories
Mr Mugabe most recently appeared to fall asleep during a discussion panel about "fragile states" at a World Economic Forum meeting in South Africa earlier this month.
There he stated that Zimbabwe was 'one of the most developed countries in Africa'
"I feel like a failure when there is this reading that the president is sleeping in conferences - no," Mr Charamba said.
He then compared Mr Mugabe to the anti-apartheid icon Nelson Mandela, whose eyes were affected by years of working in a limestone quarry while imprisoned on Robben Island.
"You were not allowed to even use flashes whenever he was in the room," Mr Charamba said.

Two men, a woman and a teenage boy suffered burns to the face, hands, neck and arms in the incident in Severn Side, Stourport-on-Severn at 07:30 BST.
The 17-year-old had significant burns to his hands and face and was airlifted to hospital for specialised treatment.
A man in his 60s, a woman in her 60s and another man in his 20s were taken to Worcester Royal Hospital.
West Midlands Ambulance Service said paramedics were called to reports of an explosion near to the Angel Inn.
The man in his 60s sustained superficial flash burns to his face as well as a neck injury.
The woman had partial burns to her hands whilst the man in his 20s had sustained superficial burns to his arms and neck, the ambulance service said.

Rebels broke into the Taftanaz base in north-western Idlib province but were pushed back by the army, rebel sources and state media said.
Reports said Aleppo airport was also under siege, and has been closed since Tuesday because of repeated attacks.
The attacks come a day after a UN study said 60,000 people had died in Syria.
A study commissioned by UN human rights commissioner Navi Pillay concluded that there had been 59,648 deaths between February 2011 and November 2012, and that figure would now have risen above 60,000.
She described the bloodshed as "truly shocking".
Syrian opposition groups had previously estimated 45,000 people killed.
The attack on Taftanaz began on Wednesday, with rebels breaking into the base and detonating a car bomb inside the grounds.
Unconfirmed reports said they had destroyed a fighter jet and a helicopter.
Further attacks followed on Thursday morning, but reports say the rebels have now been forced to withdraw. Several attempts to take the facility in recent months have failed.
State news agency Sana said government forces had "repelled the terrorists' attempt to attack the airport" and inflicted heavy losses.
More clashes were reported in suburbs of Damascus, with government troops trying to capture Daraya.
There was also fighting around the city and airport of Deir Ezzor.
Meanwhile the family of US freelance journalist James Foley have announced on Wednesday that he had gone missing near Taftanaz more than a month ago, shortly after entering Syria.
They had previously asked the media not to report his abduction.
Foley, 39, is described as an experienced war reporter. He was detained by Libyan government forces for six weeks in April 2011 while covering that conflict.

Outsourcing firm Capita was the leading gainer, up by 4% after a reassuring trading statement.
The company, which in February warned of lower margins, said it was on track to meet expectations for 2016 revenue growth. The statement was described as "unusually numbers light" by one broker.
The FTSE 100 was up 38.92 points at 6,153.73.
Shares in Easyjet, which reported a loss - largely thanks to adverse currency movements - fell 0.4% after its gain of 3.8% on Monday.
There was the usual volatility in mining stocks. On Monday the index was dragged down by these with Anglo American losing 13.8%. Morning Tuesday trade sees them recover 3.4%.
On the currency markets, the pound rose 0.21% against the dollar to $1.4438 but was flat against the euro to â‚¬1.2669.
Zimbabwe's President Robert Mugabe is not asleep when he closes his eyes for long periods during meetings but is resting his eyes, his spokesman says.
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Four people have been injured in a suspected gas explosion onboard a boat in Worcestershire.
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Fierce battles are taking place at some northern Syrian airports, as rebels try to neutralise the government's overwhelming air advantage.
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(Open): London's FTSE 100 started higher on Tuesday.