There is a phenomenon in the world of hockey, in leagues from North America to overseas, in locker rooms junior and professional, and it flourishes within this Islanders team.

That of the pregame nap.

This is not a tradition that gets handed down. It is something that merely … happens, a conclusion reached separately by, in Ryan Pulock’s estimate, “at least 75 percent of guys” in hockey, and that number does include the Islanders defenseman.

“I usually shut it down for a little bit,” Pulock said.

“I’ve done it ever since junior [hockey],” Noah Dobson said.

“I usually take short ones, though,” a laughing Robin Salo said. “Short nap. Power nap.”

Lane Lambert, a professional for six seasons in the NHL in addition to more than a decade in various leagues overseas and in the U.S., gave a smiling confirmation when asked if the nap was once a part of his day.

“It’s an awful long day if you’re up from 8 a.m. — or whatever time guys get up — through till gametime,” the Islanders coach said. “So I think just a little bit of rest can help.”

Indeed, the likeliest answer for why this is such a universal practice lies in the schedule of an NHL gameday. If a team is at home, it will usually hold a morning skate at 10:30 (this season, the Islanders have moved up their time to 10:15). If the team is on the road, it will usually skate at 11:30.

After that’s done, the game isn’t until 7 or 7:30 p.m. — sometimes even later, depending on the whims of the television schedule. Players get to the arena well beforehand, of course, and there is some work to be done pregame. But not six or seven hours’ worth.

That leaves a sizable gap in the middle of the day when players can go home or back to their hotel rooms on the road. And so they nap.

“I think the main reason is you have so much time in the afternoon,” Dobson told The Post. “Guys probably didn’t know what to do with their time, so they decided to have a nap and sleep, but some guys don’t like it. I’ve always been a guy that’s enjoyed it. It’s a good way to kill the afternoon.”

It does not seem much of a coincidence that Dobson, Pulock and Salo each said they started napping around the same time that their game-day routines started incorporating this dead time.

Even if it isn’t the primary reason, there is a physiological benefit to napping as well. The Mayo Clinic lists such gains as reduced fatigue, increased alertness and quicker reaction time. It also recommends napping in the early afternoon, which lines up well with a hockey schedule.

So it’s easy to see why those routines stay intact.

“I think there’s days that I really feel like I need it,” Pulock said. “Whether you’re traveling or you’re on the road or whatever, and you can use 30, maybe 60 minutes. But I also think it is just part of going through your routine to make sure you’re ready at night.”

The lengths of these naps are varied. Lambert mentioned 20 or 30 minutes as a reference point from when he played. Dobson prefers to sleep for an hour, or perhaps as long as 90 minutes, setting an alarm to make sure he wakes on time.

“I don’t trust myself on some days,” he said.

He has, though, always had a particular affinity for the practice.

“I used to sleep a lot in the afternoon in junior,” Dobson said. “I’ve kinda cut it down now. I’ve always been a guy that enjoyed naps, but some guys are just nappers. On non-game days, I enjoy a nice nap, too.”

A possible moment of adversity

There are not many teams whose depth would look good if three wingers in their top 12 were hurt, so the Islanders get no grief from us on that front.

It’s not yet clear whether Kyle Palmieri, Josh Bailey or Cal Clutterbuck will play on Friday against Nashville — all we know is Palmieri and Bailey are listed as day-to-day due to upper-body injuries. Clutterbuck left the Nov. 7 win over the Flames because of an undisclosed issue, and later said that it had to do with an issue that “will be with me through the season.”

At least at this point in time, there is not much to suggest any of the three will miss an extended period. And the Islanders are lucky to be playing either at home, in Newark or in Boston for the next two weeks, making it logistically easy to rely on call-ups from Bridgeport if need be. Still, this is less than ideal.

Lambert said following Tuesday’s loss in Philadelphia that he was happy with the team’s depth at forward and that playing with only 11 for much of the night did not wear down the club. As a team, the Islanders downplayed the notion that a 15-game November was the cause of their tired performance in the 3-1 loss to the Flyers.

All of that can be true, but as the Isles enter a tame three-game stretch at home, it’s not a great position to be in. If they fail to pick up enough points against the Predators, Blackhawks and Blues, there could be trouble on the horizon. Those three matchups are followed by a back-to-back at New Jersey and home to Carolina (Dec. 9 and 10), before a trip to Boston (Dec. 13) and a Western swing through Arizona, Las Vegas and Colorado (Dec. 16-19) complete an utterly brutal 11 days.

An encouraging start

Although Simon Holmstrom was set to be a healthy scratch Tuesday before Bailey exited following pre-game warmups, it is telling that in his first four games, Lambert has not taken Holmstrom off the ice late in the match.

Holmstrom has not by any means played a heavy workload — he averages 11:34 of ice time — and he took only one fewer shift in the third period on Tuesday (six) than he did in the first or second (seven apiece). Ditto for Saturday, when he took five shifts in the third, compared to six in the first and second.

Reviews of the 21-year-old Swede from Lambert and his teammates have been positive, and in a conversation with The Post following Tuesday’s morning skate, Holmstrom said his NHL stint has gone well.

“Go out there and play with confidence and play my game, I think that’s the biggest thing,” he said. “And all the small details that you really gotta do out there. I think those are the biggest things.”