Dawn is still an hour away, and the brown clay of the training area is dull and matte under the stark lights. The air carries the characteristic tang of sweat, musty earth and sweet hair pomade.
A couple of dozen prodigious young men, naked but for a diaper-like loincloth, go through their exercises. They raise one leg sideways high into the air. They crash it down again. And as foot stomps clay, they exhale volubly, giving the place a sound reminiscent of an old railway station, complete with a steam train impatient for departure.
Along with sushi and geisha, sumo wrestlers are one of the most readily recognised images of Japan, the glaring physical exception in a country that otherwise is partial to the neat and dainty.
And the place where every sumo wrestler learns his craft is a training hall like this one, Musashigawa, located - as are most other halls - within striking distance of the sumo stadium in Tokyo's Ryogoku district.
The sumo day starts early. I got to the training hall at 5.30am, shortly after practice had begun. The front door was shut and nobody was around, so I assumed that one simply marched right in. So, I did - and found myself in the training area, surrounded by semi-naked wrestlers. They eyed me with bewilderment until a young wrestler of about 16
politely showed me the way to the viewing area on the other side of the hallway.
Sumo training halls (sumo-beya), known as stables in English, operate as combined practice and living facilities. Despite the name, there is no horsing around. The coach barks out orders with the kind of voice you normally hear in yakuza films. Like the rest of sumo, the stable is a serious, unsmiling, feudal world.
Seniority rules in sumo, and at the top of the pyramid in the stable is the oyakata, the master. Being the boss, he doesn't show up until after 7am for the daily morning practice. When he does arrive, all the wrestlers approach, bow deeply and greet him. But he studiously ignores them, just as he ignores me, when he enters the tatami-mat viewing area.
A few minutes later, the massive bulk of 'Moose' Musashimaru enters. Moose, weighing in at 230kg, recently announced his retirement as yokozuna, sumo's grand champion. If the yokozuna are at the peak in the sumo world, those in the trough are the hapless junior wrestlers, who have to perform menial tasks for those above. Life is harsh for the juniors.
'Sumo' is written with the Chinese characters that signify 'mutual bruising'. With juniors, though, the bruising generally goes in just one direction. I watch as an ill-tempered senior exercises his obvious fondness for dispensing kicks and slaps on younger wrestlers. Coaches are not averse to using bamboo swords to get their point across to slow learners.
There are about 800 sumo wrestlers in Japan, divided among 54 stables. A huge change takes place when a wrestler leaves the junior ranks and makes it to sekitori, the professional rank. As sekitori, a wrestler acquires his own groups of fans and supporters. He also acquires a great deal more financially.
The highest junior rank may get paid just $4,500 a month, but at the lowest sekitori level he earns $67,000. The yokozuna have a $200,000 monthly salary,
plus hefty payments when they win bouts.
By about 10am, after the five-hour morning practice is over, it is time for the all-important business of eating, as wrestlers sit down to their daily brunch of chanko-nabe, a high-protein stew containing meat, fish, chicken and vegetables. The chanko-nabe helps build sumo-size bodies - that and washing it all down with copious amounts of beer and having a good sleep afterwards.
As with all things, the juniors serve the sekitori and don't get to eat until the seniors have had their fill.
Practice is usually held from 5am to 10am, although you can arrive and leave when you wish. Four good stables to try are Musashigawa (tel:  3801 6343, nearest station Uguisudani); Azumazeki (tel:  3625 0033, Honjo-Azumabashi station); Oshiogawa (tel:  3643 8156, Kiba station); and Oshima (tel:  3632 6578, Ryogoku station). To arrange a visit, ask your hotel concierge to phone in advance and obtain directions.