Noise margin goes down during the day.

I've recently setup a routerstats program for the HH5 and one of the things that seem to stand out is how the noise margin seems to gradually go down during the day. Then at night goes back up again.

I think I'm right in thinking that the lower the noise margin means the lower the speeds will be?

So what kind of things can kick in during the day to be doing this?

At 4pm the downstream number is 5.2 db, and the upstream number is consistant at 6.2db.

The noise margin will fluctuate. As long as the connection doesn't drop then it won't affect sync speeds. If it is going down to 5dB then unlikely to cause any real issues at all - if it was down to 1dB or thereabout I would start to think it might have some effect on errors and could kick in DLM. From what you have said I think what you are seeing is perfectly normal and not of concern.

Except we would all expect the noise margin overall to be higher during the day than at dawn/evening/night times.

Mine is the same, 3 to 5 during the day and higher at night and my line is erroring,

All depends on the local area, e.g. industrial/heating issues can override the atmospheric effect of the sun setting

That's true.

As RobertoS points out, another way of looking at it is the margin goes UP in the evening.

The noise margin varying over 24 hours isn't usually anything to worry about. Big swings occurring frequently are usually best looked into.

This write-up tells you what it basically does. That was written for ADSLx but the same principles apply on FTTC.