After I had been mentoring meditation students for a few years I noticed a pattern: it is not uncommon to experience a “dip” after roughly 40 minutes of sitting silently. You get started, mind settles, you’re in the groove, and then tiredness, or irritation, distraction, or some from of “malaise” sets in at the 40 minute mark.
Recently I had a conversation with an apprentice which went something like this:
Apprentice: “I’ve been sitting daily for nearly six months now and I think I’ve got opening awareness down pretty well. What should I do next?”
Me: “How long is your daily sit?”
Apprentice: “I’ve gotten up to 45 minutes. Sometimes I only do half an hour, or twenty minutes if I’m not in the mood.”
Me: “Have you tried increasing to an hour sometimes?”
Apprentice: “I don’t see that would make any difference.”
Me: “Did you try it?”
Apprentice: “No, but I don’t see why it would make a difference?”
Me: “It’s worth asking why are you stopping at 40-45 minutes? Many meditators plateau around 40 minutes, because there can be powerful unconscious resistance to going longer. It’s a common, natural pattern for your mind to start getting restless and distracted around 40 minutes in. Then you think “OK, I’m done now, time to think about something important.” It seems like enough. Nothing is happening, it’s gotten boring, you’ve done your daily thing, it was fine, you achieved some awareness of your thought processes, your mind was relatively quiet, you nailed it, let’s move on. So sitting for an hour means pushing through that barrier. You might find it surprisingly different to sit regularly for an hour.”
Apprentice: “How?”
Me: “Well, try it and see! You can do it. It’s only fifteen minutes more.”
I don’t want to preempt your experience. Although this seems to be a pattern it doesn’t apply to everyone. However, if you are a 30-40 minute sitter it might be interesting to notice what is causing your sit-length boundary. Pomodoro sessions are often set between 25 and 45 minutes for a reason: the brain seems to have a natural cadence of initial engagement in activities. However, “flow” states tend to occur outside of this container over longer time periods. Your natural, initial attention span might be an obstacle to more stable, effortless, and clear states of mind in the long run.
Anecdotally, many meditators do find that in the last ten minutes of an hour their mind gives up on the restlessness and settles down differently. The mental detritus thins out, and you may find yourself in a cool, clear, still pool of awareness. Once you are familiar with this mind state it becomes more available at other times, on and off the cushion. That’s the purpose of the practice.