Could there be any reason?

[talos]

Over the last few weeks I have noticed my speed dropping, a fairly steady decline, because I am not a gamer I did not notice too much, but when I clicked a link on here that took me to a video which stuttered I decided to investigate, my speed had dropped to about 400kbps .
    I waited until the next morning, early, to run a BT test got this result       DL 700 kbps acceptable range 200-1000
                                                                                                        UL 448kbps   IP Profile for line 750kbps
    Now I leave my router switched on 24/7 because I believe this kept it stable,  I had a general clear out, cookies, history Etc and rebooted my router, none of which seemed to make much difference, by now it was getting late so I switched all off, inc router until next morning.
  I did another early morning speed test and to my surprise I got 1396kbps Dl acceptable range 400-2000,  Dsl Conn rate 1792 kbps Down 448 Up,  Profile 1500,  all of which means I am back to normal. Now to the questions,  What might be the cause,   Would it be better to switch of the Modem/router at night and are BT just playing with me?

[Rik]

It looks like your line is picking up a lot of noise from somewhere during the night, Bob, forcing low speed syncs and a low profile. If the source isn't local, ie you've done all the usual tests, including the de-tuned AM radio, ask IDNet to investigate.

[talos]

Thanks Rik, as an experiment I left it on last night, and all was well this morning, the speed doesn't worry me much usually, but it felt like I was slowly being turned down until I noticed, I know a line can become slower due to congestion at peak , but could that lower the profile?

[Fox]

It looks like your line is picking up a lot of noise from somewhere during the night

Probably Rik's snoring  

I would keep an eye on it, it could be a one off event (BT trying to fix their plumbing again?)

[Rik]

I know a line can become slower due to congestion at peak , but could that lower the profile?

No, only a low sync can do that, Bob.