"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.
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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.
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.