— -- Navy SEALs may be some of the toughest people on the planet, but even an elite soldier doesn't do well without sleep.

Stew Smith, a former Navy SEAL, said he survived for three days on no sleep before the hallucinations started to set in. After about 72 hours of sleep deprivation during training, Smith recalls mistaking an airplane for a flying horse, perceiving a bridge as a giant Pez dispenser, and seeing a squat, muscular body builder where there was in fact a fire hydrant.

But he wasn't quite done. Smith and his crew had another two days of running, swimming, paddling, climbing and plunging into freezing water. In total, he and his team had to stay awake for a punishing five days as part of their Navy SEAL training.

“I would be thinking of something and I would see it in front of me like a cartoon character,” he recalled, describing his hallucinations. “When you’re losing sleep, after a while you turn to this fight-or-flight response. You just go into survival mode.”

At the time, Smith said he survived by staying in constant motion, staying uncomfortable, and psychologically breaking each day into a series of six-hour stretches until the next meal.

While SEALs may need to stay awake for days in life-or-death situations, Smith said he would never wish this kind of sleep deprivation on anyone. ABC News' own Dan Childs, head of the Medical Unit, is currently staying up for 40 hours as part of the "Good Morning America" 40th anniversary event to show how important sleep is. That's half of the time Smith was required to stay up.

Dr. Kirk Parsley, another former SEAL who is also a physician specializing in sleep health, couldn’t agree more.

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While SEAL training didn’t break him, Parsley said he was almost brought down by the chronic sleep deprivation that accompanied his time in medical school and residency.

“I had extreme chronic anxiety, I was super emotional and impulsive, I couldn’t manage my appetite or keep a good workout regimen,” he said. “All the metrics people measure their success by ... I was suffering in all of them." It wasn’t until he began prioritizing sleep that he felt like he regained control of his mental and physical wellness.

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