When to avoid crowds on trains at tokyo?

Hi,I'll be traveling to japan with my sister this coming January and was wondering if anyone knows what are the times whereby the train around Tokyo will be packed both morning and night? Just wanted to have a rough idea so i can avoid huge crowds. The packed trains seem pretty intimidating to me.

Here you are:

http://www.japan-guide.com/e/e2020.html

Thank you very much.
I tried searching it on Google and here but that didn't pop up.
Do you know whether it's the same for weekends?

I would not travel on a train around 4:00-6:00 most people are getting off work. I went threw Tokyo station with a big bag around 5:00 and it was so hard! People would not let me by or help me with my infant son--and I was alone!

I call the times I've been on really crowded trains "Gewaltkuscheln", which could be translated as 'violent cuddling'. ^^

In general, on quite a few trains, it's difficult to get a spot to sit directly after boarding the train even when it's not so crowded.
If you avoid the rush hours in the morning, it's ok, but usually throughout the evening it gets a bit crowded on most lines.

So far I've only been one time in a train where I couldn't move anymore due to the people to space ratio being shifted to an uncomfortable level. ^^

Do you know whether it's the same for weekends?

Unlike on weekdays, there are no excessive crowds on weekends.

Thanks everyone for your advice.
I really want to avoid the crowds. Now i can start planning my trip.

From my personal experiance the trains start getting really crowded (meaning you may have to push your way on if you're in the back of the line at the platform) around 17:30 and it's like that for about two to three hours.