A couple of questions about CBT

Hi everyone, I wonder if anyone can give me some advice relating to CBT.

I have been suffering from depression for many years and I decided to read David Burn's book, Feeling Good. I've been working with the Daily Record of Dysfunctional Thoughts, where I write down each unpleasant emotion, cognitive distortion and finally create a rational response.

However, I have a couple of questions:

At one point in the book, he talks about writing down unpleasant emotions as they occur (at the time), later he mentions spending fifteen minutes at the end of the day doing this. On the one hand, it's not always convenient (or even possible) to go through the process of writing down and assigning rational responses at the exact time that each unpleasant emotion occurs, but on the other hand, if I leave it to the end the day, I'm not dealing with the irrational thought as it occurs. What is the best way to approach this tool?

Also, my first instinct is that sometimes unpleasant emotions can occur without being preceded by irrational thoughts. Is this possible? Sometimes it's possible for me to overlay a fairly general or abstract thought onto the emotion, but I fear that I'm just superimposing a thought onto the emotion, and in effect reversing the order of this exercise. Are unpleasant feelings always preceded by irrational thoughts, and is there an effective way to identify these thoughts?

Many thanks, Simon