Given the information you have provided, dare to say, don't disclose anything unless it may impact your performance on the main job.
The key to good relationship is openness. However, your private life is yours, and you are not obliged to let others in.
There are serious exceptions to that. I'm not sure if they apply to you, but the others who may read this, should consider those very carefully:
- Once you sign a working contract, you write all prior inventions. They are specifically for the purpose of protecting you against any accusations. Here's a good article to learn from.
- Your private life should not violate your prior agreements on your job - NDA, non-competing, etc. If the side project is in a similar business area or uses some knowledge you have obtained while working in your company, please stop and consult your lawyer.
- If you work on a side project till late, you may simply not have enough time to sleep. This will certainly affect your performance during daytime. This seems to be a good reason to ask for fewer working hours.
- Avoid spending even a tiny fraction of your work time for a side project. It sounds trivial, but hard to follow.
One can't sail on two boats. Sooner or later you have to choose. I've seen a few people who constantly have side projects, but they all are highly motivated and self-organized. They just can't stop inventing. You know better who you are.
You should also prepare for accidental disclosure. Make sure you are absolutely clear on your main job so that nobody even suspected you wasting business time for a side project or violating any rules. Depending on a country, you may risk a legal prosecution.
Also, there are several discussions you may find useful: one, two, three. Notice there is no agreement on this subject, so please consider everything above as a suggestion, not an ultimate answer.