Yes - another novice question from me :(
Our brew bubbles away through the airlock for most of the day, but when I come down in the mornings it is dormant (no bubbles for the 2 minutes I was looking at it). There is a 4 degree ambient temperature difference in the room between day and night (18C - 22C / 66F - 72F). Would that be enough to cause the brew to go quiet over the night time? Should I be wrapping it in a blanket
Thanks for the help!
Assuming that you are only seeing a four degree ambient temperature change, I would imagine that your little yeasties are falling just short of their "sweet spot" overnight, but would not worry that there is anything to be alarmed about. Fermentation is still carrying on, but just not at the same levels overnight.
Even with an ambient air temperature difference of four degrees overnight, I doubt that you are seeing a full four degree swing in the temperature of the fermentation, as the amount of energy (or loss of energy) in the form of heat is not instantaneous. When all is said and done, I would not foresee there being enough of a swing to actually impact the final product. In the greater scheme of things - and as Graham points out, depending on the style - you will still end up with something tasty, most likely.
As you get further along into the obsession/hobby, fermentation chambers go a far way to help control temperature, depending on the requirements of your brew and the design of the chamber you pursue, you will be able to create more reproducible results while minimizing the influence from external factors (i.e. controlling temperature and light. As far as a blanket goes, I used to use towels early on, but that was more around controlling exposure to light in my glass carboys. From your comment to your original post, I am assuming you are using a bucket, which should be fine on its own out of direct sunlight.
I have personally found that a lot of yeasts are surprisingly sensitive to ambient temperature. I have particularly seen what you describe in what I might think of as the shoulder season, particularly fall. It is almost certainly temperature-related and a few degree temperature difference can cause some yeasts to slow down dramatically. This isn't always a bad thing unless you are impatient.
My last batch of mead (an acorn mead, as I am always trying odd things) was started in October and never really bubbled as I would have expected. I left it in the carboy for about 9 months and finally bottled it. It was good but not for the impatient!
So my thinking is that the temperature is low enough that the yeast are sort of semi-dormant at night. They are still fermenting your beer, just a bit slower. Be patient.
I'd say this was probably a coincidence, if anything. Don't worry about the blanket, its not needed unless this beer is a Saison.
And a reminder, the fermentation is likely taking place at several degrees higher than your ambient temp. So if you have ambient temps of 70F, the beer is likely 75F or higher if this is the first week. You'll want to bring that down nearer to 65F for virtually all ales for better quality beer.
