How to remove dullness of mind, without sitting meditation, in regular activity?

As per me, my personal observation is: mind is delicate but not stable. I meditate to stablise it by performing different-2 tasks in my day to night, unending actions. When i sleep i meditate with closed eyes & slow working senses 'but mind do not sleep and get stablised itself. I have nightmares and some precious dreams. What i get is 'what i concentrate', a perception against distractions, pressing my will in favor of my belief , i start thinking (create a perception) that i am meditating. When i convince myself that my dullness is my own perception, that very moment i start meditation by taking the advantage of my illusion-free mind(as all the senses are asleep), 'that very moment i feel good, breathfree, out of the body. It is true that mind never stablises itself, it is true that mind creates a problem by creating a perception(while editing my perception), it is true that i am able to make my mind to concentrate on my belief. It is true that i can practise buddhism or any kind of perception or do meditation, everywhere, anytime. Even now, actually in present.

A lot of good answers above. A few extra points to consider:

Being really tired, a lot of lay meditators prefer to meditate in the morning. Get to know your circadian rhythm.

Lack of quality sleep, due to excessive snoring or sleep apnea.

Being too full, difficult to meditate on a full stomach.

The environment is not conducive, too dark, too much noise or distraction.

Ill health (diabetes, HBP, etc), obesity, nutrient deficiency.

Psychological reasons - e.g. depression.

Medications and intoxicants.

Lack of interest in the task at hand

Lack of purpose or understanding of the task at hand.

Dullness of mind is associated with sloth and torpor. When this happens you feel:

• sleepy or drowsy
• heavy

This itself is a sensation. Concentrate on the impermanence or arising and passing of this sensation. When you have this Ignorance dominates and to overcome this when sensation arises contemplate is impermanence, dispassion, ending and letting go as per advise in Pacalā Sutta. Similar advice is also found in Avijja Pahana Sutta 2, Pahāna Sutta, etc. As in the case of Pacalā Sutta you can note this but best is to avoid mental verbalisation of which is discussed in section 4 Note on noting in Giri-m-ananda Sutta commentary by Piya Tan which takes a more lenient stance; S.N. Goenka like the Thai Forest Tradition on verbal noting take a more hard stance on verbalisation or visualisation which I feel is right.

"Nothing is worth clinging to"

When this was said, the venerable Mahā Moggallāna said this to the Blessed One:

"In what way, bhante, in brief, is a monk freed through the destruction of craving, that is, one who has reached total perfection, the total security from bondage, the total holy life, the total consummation, the highest amongst gods and humans?"

"Here, Moggallāna, the monk has learned that nothing is worth clinging to. And, Moggallāna, a monk has learned that nothing is worth clinging to, thus: he directly knows all things [he directly knows the nature of the all]. Having directly known the nature of all things, he fully understands all things.

Having fully understood all things, he knows whatever feelings there are, whether pleasant, painful or neither painful nor pleasant. As regards to those feelings, he dwells contemplating impermanence in them; he dwells contemplating dispassion [fading away of lust] in them; he dwells contemplating ending (of suffering) in them; he dwells contemplating letting go (of defilements).

When he dwells contemplating impermanence in them, contemplating dispassion in them, contemplating ending in them, contemplating letting go, he does not cling to anything in the world. Not clinging, he is not agitated; being not agitated, he himself surely attains nirvana

From meditation instructions we know that dullness happens when you suppress (forcefully restrict/constrict) yourself too much. This usually happens when you try to focus on task that has absolutely no connection with any of your goals and interests.

In meditation the usual advice is to "expand" your awareness a little to "connect" your activity with your surroundings and your goals. "Where am I, what am I doing and why?" Sometimes we need to actually go as far as to deliberately think through: "I am doing this because it will lead me to that; I need that because I want to get so and so" - etc.

Then once you have a notion of why you're doing this and how it fits with your bigger goal "loaded" into your immediate awareness, then it makes more sense to your mind - and dullness will be replaced with either a sense of gladness (if you know you're making good progress) or with a sense or worry and urgency (if you know you are not making progress).

So you can generate excitement by thinking of a realistic goal that is not too far, and thinking how you are making progress. Or you can generate urgency by thinking about a bigger goal and about the danger of what will happen if you don't make it.

By connecting what you are doing with what's important to you, you can overcome dullness. By carefully choosing the background narrative, you can control your state of mind.

You can indeed meditate(Vipassana) while performing any task. ex: if you are sweeping, keep reminding: sweeping.. sweeping.. sweeping..

Dullness or boredom is a form of aversion. Ignorance is the cause of it. To overcome dullness, simply note it as it is without taking it as "I am bored" or "it is boring".