Others say he only made his mind up over breakfast the following day. So let's just split the difference and imagine the decisive moment came when he woke up screaming in the middle of the night.
Goodness knows, he had enough reasons to after a November Test series that must rank as one of the most maddening any coach has had to endure. There had been moments of optimism, for supporters as well as Robinson, against New Zealand and South Africa, but the wheels fell off against Tonga in a performance of truly gruesome profligacy.
Most frustrating of all for Robinson was the fact his players had delivered an almost equally wretched display against Italy in their final match of the last Six Nations Championship. It has become customary in recent years for Scottish teams to end campaigns on a high, leaving some scope for optimism, but lately they have taken to throwing themselves off a cliff in their final outings. Small wonder that Robinson felt he could do no more for the team.
The players have mostly kept their own counsel, but rumblings have been heard. Some, it is said, found Robinson difficult to deal with, dissuaded from casual approaches by his brooding intensity. Those losses to Italy and Tonga cannot be explained as a few individuals having off days, rather as massive collective failures. Their expressions of self-reproach over the last week have had a suspiciously dutiful tone. How many of them are really sad to see the coach go?
Time will tell if Robinson's legacy is to be positive. It is something of a standing joke in rugby that a deposed coach should subsequently try to claim credit for any subsequent successes his former charges enjoy – it is known as doing a Matt Williams – but if results give Robinson grounds for going down that route then his case will be stronger than most.
Certainly, he hands on a side that can look after itself. The first half of the South Africa match was exceptional for the fact Scotland could not live with their opponents' physicality; how easily we forget how routine such things once were. Robinson has brought through a crop of promising players and taught them a way of playing that, if it ever does click, could see them blow teams away. And yet, there are still critical issues. The fly-half question is the hardiest and thorniest perennial in the Scottish game, and we are no closer to having an answer after three games in which Greig Laidlaw seemed to take a backward step after all the progress he had made earlier in the year. The chaotic closing sequence to the Tonga game must also raise questions about Kelly Brown's leadership of the side.
Whoever takes over from Robinson will hope for the kind of kick-start effect that Robinson and his pre-decessor, Frank Hadden, both enjoyed. Robinson won admirers and bought time with an IRB Nations Cup win in his first month in the job, then cemented his gains with a dramatic Murrayfield victory over Australia a few months later. His players looked eager to please, and his successor may just enjoy something similar when the Six Nations begins in two months.
Having paid off Robinson, albeit with nothing close to the full value of the remainder of a contract that was due to run until 2015, the desire for continuity is not the only reason for the SRU wanting to appoint a caretaker who is already on their payroll. The likeliest men for that role are Sean Lineen and Scott Johnson, although the appointment of either would beg questions about who should look after the forwards.
Johnson has a poor track record as a head coach. Lineen left Glasgow with a reputation stained by his team's inconsistency. Either would need a helpmate who could instil some hardness up front.
And in the longer term? The most significant part of that jigsaw falls into place when the 2015 World Cup draw is made in London tomorrow, with Scotland among the third seeds. The permutations are potentially horrendous – New Zealand and England anyone? – but a lot of water can still flow under the bridge before that tournament takes place.
Let's just hope that whoever does succeed Robinson on a long-term basis does not spend his days trying to push the stuff uphill.