Changing Ip address

I am on BT ADSL with about a 2meg connection.using a HH4. The rate is not an issue as I am in a rural area an in someways lucky to get that.
I know my package does not have fixed Ip address but my ip address stays the same for 4 weeks and then always changes on wed/thurs night.
Can those with much more technical knowledge explain what triggers this change of ip address and what bt gains by doing it?
Thanks

Do you loose sync at the time it changes? If so, is it anything you do than triggers it?

BT gains nothing from the change it is just that you are allocated the next free address in those owned by BT and your previous one is recycled for another user.

It will normally happen when your router disconnects and reconnects to the internet. Doesn't the BT Home Hub reboot every four weeks anyway?

In reply to a post by RobertoS:

It will normally happen when your router disconnects and reconnects to the internet. Doesn't the BT Home Hub reboot every four weeks anyway?

Every two weeks.

Which makes his 4-weekly IP address change odd.

I used to be with O2 before they disappeared and their ip addresses were very 'sticky'; often I kept the same address for months on end. Now I am with BT, on FTTC, and my address seems to change more often than once per month, although I haven't checked it accurately, I just looked at my TB profile speed test history.

In reply to a post by RobertoS:

Which makes his 4-weekly IP address change odd.

Yep. BT have never explained the circumstances surrounding why some/many/all Home Hubs reboot every two weeks, so it's possible it doesn't affect everyone.

Many thanks for all the replies.
It definately happens on a four week cycle and always happens in the early morning between 0100 -O400 and has done since I started checking last September.
Does anyone know if the HH3 has the same reboot cycle?

In reply to a post by crule:

Does anyone know if the HH3 has the same reboot cycle?

Ah, not sure about that. The reports I have seen have about the 2 weekly reboot have been the HH4 and HH5.

It doesn't affect all, I have a HH5 on Infinity2 at my parents and I have seen connection times of 20days+