Take the fairly simple example of reading: All you have to do is look
at a series of words and understand most of them. Still, when you
start reading a book, or resume reading one that's lain neglected for
a few days, it often takes a few pages to “get into” the book. Having
to persist for a while to become immersed in your reading can be
attributed to two things:

One is the development of easy concentration, which comes from giving
most of your attention to reading until it seems more natural to
continue than to stop. This can happen with almost any activity, if
you persist at it for a few minutes.

The second thing that takes time is increasing your awareness of the
relevant subjects, such as the characters, plot points and places
represented in the text. As you read more details of, and
relationships between these different elements of the story, the ideas
become clearer and more prominent in your mind. If you were already
part way through the book then this process will also include
remembering some of the things you've already read.

So to get caught up in reading a book you need to pay attention until
it seems more natural to continue than to stop and you need to build
up an awareness of the relevant ideas until they dominate your
thoughts. These two processes apply not just to reading but to
anything that requires paying close attention, such as writing, office-
work, watching a movie, playing a computer game or having a
conversation. They allow you to become more immersed in and effective
at the task.

Unfortunately it doesn't always work out this way and you don't always
reach a high level of effectiveness. The two most common reasons for
this are too much distraction and breaking too early. If you try to do
too much at once then you won't do anything well, and if you stop what
you're doing just as you're starting to get into it then you will lose
the awareness and concentration that you've built up instead of using
it to get the most out of the activity.

Some things are more distracting than others. For example if you're
watching tv you could eat a meal at the same time, or roll up
newspaper for starting a fire. These things won't stop you paying
attention to the tv show, but if you start reading a book then either
the tv show or the book, or both, will suffer from a lack of
attention. You won't be able to enjoy both things properly and you'd
be much better off doing one and then the other.

Besides stopping too early or being too distracted, the task might be
just too difficult. If something is way beyond your current level of
skill or understanding then you might get more frustrated as you go,
rather than more effective. In this case you could do something of the
same kind but a lower difficulty. This will allow you to improve your
skill and knowledge and over time you can work your way up to more
difficult levels.

So the main factors that determine how quickly you improve at a task
are your level of concentration, how long you persist for, and the
difficulty of the task relative to your own ability. These are not the
only things. For example if you are too tired or intoxicated then your
brain won't work properly and improving its performance will be much
harder.

No matter how fast or how far your improve, though, eventually you'll
start to get worse. After maybe an hour or two, concentrating will
begin to fade, the ideas will start to get less clear and new
information will be harder to process. While this might feel similar
to the difficulties that you experience when first beginning a task,
they arise from different causes. When you start to get less effective
at a task, the key to improving again is not to persist as when you're
first starting out, but rather to stop for a while.

The decrease in your attention and ability comes from either tiredness
which can happen no matter how skilled, fit or smart you are, or from
boredom which can occur no matter how much you enjoy the task to begin
with, or from the increasing urgency of other things that need
attention.

If there's a strict deadline that won't be easy to meet, and you're
not going to endanger anyone, then it might be worth persisting for a
while even after you feel your attention start to wane. You might get
more done and then when you finally do take a break, you will have
learned more for next time you start. If on the other hand the task
isn't urgent or you can afford to take a break, it's worth taking one
soon after you start to drop from your peak effectiveness. That way
you get the maximum enjoyment from the activity, and you have used
your time most efficiently.

Becoming bored with or tired of one task doesn't mean you will be
bored and tired with other tasks. Boredom can often be cured by doing
something else and tiredness can be limited to specific mental or
physical faculties that have become exhausted by the present activity.
Unless you need to sleep or are exhausted in many different ways,
there is probably something else you can enjoy and do effectively. If
not, then the sooner you stop and have a proper rest, the sooner
you'll be able to return to the task with new energy and a fresh
perspective.

While intoxication or lack of sleep are more obvious causes of poor
performance, the more subtle ones can affect us more because they're
easier to miss or forget. Using your time in a way that respects your
brains limitations, but also takes advantage of its tendency to adapt
to a task, helps you get more from each minute you're awake.