As it is winter, even by keeping my bonsai Zelkova indoors, it losts most of its leafs. For a week or so, I let it outside during the day for it to take the sun.
Now, it's already producing new leafs.
Is it harmful for the tree to not let it "sleep" in winter and to have it produce leafs all year long? Can it be exhausting for the tree?
Japanese elm is a deciduous tree and, as such, it expects to lose its leaves and become dormant during winter. It is an outdoor bonsai, but may be susceptible to frost damage when the weather is very cold because of its shallow root system and container, so is best moved to a cold greenhouse or somewhere that isn't warm, but frost free. It's not good to keep it in a heated environment during winter, which it sounds like you're doing. If you can find somewhere unheated but frost free with daylight, move it there instead. Further information here https://www.bonsaiempire.com/tree-species/zelkova
I am worried that you are taking this plant indoors and outdoors willy nilly. Without acclimatization this will kill plants.
A Bonsai is even more dependent on the environment it gets used to. All plants in pots are far more susceptible to cold because the roots are in a pot and roots are the most fragile part of a plant and cold will kill plants in pots. Especially plants with abnormally few roots in abnormally little soil. I am amazed your Bonsai is still alive.
Don't worry about the dormancy of this species, it doesn't take much to promote a mitigated dormancy. Getting your plant out of the sun during the winter (we are talking about indoors) out of the sun coming through a window in a cooler part of the room is enough.
Do not fertilize at the beginning of a winter, that is for sure. Fertilizer should happen in the spring is you aren't using an extended release fertilizer. An extended release fertilizer will not allow chemistry out of the little pellets when the soil is cooler than normal. Just do not add any fertilizer.
Please send a picture of your Bonsai. Bonsai deserve more involvement than other plants. They NEED more involvement than other plants.
What is your watering method? What fertilizer have you used? How long ago? Have you transplanted this Bonsai yourself? What soil have you used?
Stop taking this plant out of doors. Keep it stable, it is in dormancy after all. It doesn't have to lose leaves to be in a dormancy. No fertilizer. But continue the watering which if not everyday then every other day at the most.
Get your Bonsai out of direct light coming through the windows and find a place very close by that is cooler...in the kitchen? Next to the mud room door? But do not put it in the garage. Just a bit less light and a bit less water and no fertilizer and a bit cooler is plenty to accommodate dormancy of this Bonsai.
In the summer when there is no chance at all of freezing or vast temperature changes, a covered porch or covered patio is perfect. Doing this for a summer will rejuvenate all indoor plants. Plenty more light to make healthy roots and healthy carbohydrate storage to last the winter... without having to acclimate the plant to sunlight at the beginning of summer and then re acclimate that plant to the indoors in the late summer. Don't be taking this indoor adjusted Bonsai out of doors and into the sun.
Plants in pots are fragile and very sensitive to the cold temperatures because the roots are OUT of the ground. Bonsai, although demanding, are so very rewarding. Only real gardeners are able to keep a Bonsai alive. And I've killed at least two! WAaaaaah! grins!!
