Look at the Olympics, then look at the World Cup, athletes often arrive at the host city one week or more in advance to adjust to the time difference, adapt to the environment, and adjust their state.
However, the NFL needs to accomplish this task during a dense schedule of regular games, a difficulty that can be imagined and might even far exceed expectations.
Generally speaking, when the NFL travels to London or Berlin for overseas games, they need to start adjusting on Monday according to the plan of the physical trainer, adjusting ninety minutes to two hours each day to ensure players gradually adapt to the time difference across the Atlantic without realizing it.
When they travel to London, the time difference is already adjusted to London status, and once the plane lands, players can seamlessly start training.
