Before other replies come in, you might be interested in my earlier response concerning the work regimen of the agricultural workers who have formed the bulk of the workforce in pre-modern and early modern era.

The assumption you made in the original question is basically correct. It also resembles a rendition of the commonly cited quote by James Pilkington, Bishop of Durham who wrote the following somewhere in 1570s:

The labouring man will take his rest long in the morning; a good piece of the day is spent afore he come at his work; then he must have his breakfast, though he have not earned it at his accustomed hour, or else there is grudging and murmuring; when the clock smiteth, he will cast down his burden in the midway, and whatsoever he is in hand with, he will leave it as it is, though many times it is marred afore he come again; he may not lose his meat, what danger soever the work is in. At noon he must have his sleeping time, then his bever in the afternoon, which spendeth a great part of the day; and when his hour cometh at night, at the first stroke of the clock he casteth down his tools, leaveth his work, in what need or case soever the work standeth.

Now, although taken out of context, this might seem as the corroboration of the notion that people in the past had relatively lax working regimen, it needs to be stressed that this particular passage is a part of the bishop's condemnation of general laziness he accuses his contemporary people of. In the verses than follow he bemoans the difficulty of finding a good, diligent and dedicated servant, or criticizes an average soldier who, after his daily duties are done, goes 'to refresh himself with gaming, swearing, whoring or else he thinketh himself nobody: he thinketh it shame to live honestly in order'. This strongly suggests that the passage quoted above should be taken with a pinch of salt at the very least, much like various allegations about 'the youth today'.

But in general, the work in pre-Industral world was relatively lax and slow-paced, chiefly because it was not done for the purpose of 'doing work' but rather to finish very specific tasks, with the time being spent on them being of no concern. Peasants and artisans usually did not have a concept of 'working hours' as they were simply doing what needed doing and if there were no tasks for a day, that meant end of work (craftsman, of course, would hang around his workshop in case some customers come in). Of course, if a craftsman had a large and urgent commission or the weather was unfavourable and harvest had to be done as soon as possible, this could have meant a very intense work, but these were exception rather than rule and the work itself was inalienable from the results. Necessity to provide everything though one's own work in the case of meany peasants, as well as the relative scarcity of currency on one hand and the relatively small trading networks meant that people were largely doing what they had to do and not more. There was no point in making the output larger if the surplus products could not have been easily sold and the time spent on such work meant less time to be spent on other house chores that were equally important to the quality of life. A relative slow-paced approach to work seems to be corroborated by the actual demands of the workers to increase their work time (similar to modern people asking to work overtime), as exemplified by the petition of fullers' assistants in Arras in 1315.

Work has largely been conducted from sunrise to sunset. As le Goff notes, work during nighttime was considered somewhat of 'urban heresy' as it disrupted the course of living for other people because of the high concentration of the workshops that were usually located in the same buildings as living quarters of the workshop owners. This meant that any work, especially loud one would be a nuisance for the neighbours and thus was avoided either by common agreement or enforcement, although the exceptions existed, such as the ordinance permitting some Parisian crafts to operate until nightfall that was issued by Philip IV and subsequently introduced by provost Gilles Haquin in 1322. Likewise, in the end of the 'long 13th century' around late 1320s we can observe the emergence of 'work bells' that were installed in the municipal belfries and were used to announce the starting and end of a working day as well as to signal the breaks. These were generally installed in cities with a well-developed cloth industry (in a wide sense of this word, as the production itself was not performed on industrial scale) that required large number of assistants and semi-qualified workers. But in general, the work day for anyone started at sunrise and ended at dusk, as explicitly stated in the 1395 ordination from Paris: 'the working day is fixed from the hour of sunrise to the hour of sunset, with the meals to be taken at a reasonable times'.

Thus, the actual work of the agricultural workers was largely determined by what was to be done, and as it was dictated chiefly by the natural cycles (of course, we need to remember that 'winter' could have differed between 'Medieval Optimum' and 'Little Ice Age'), the shorter days in winter corresponded to the lower amount of agricultural work that was to be done. Winter was often considered to be a time of rest (lower availability of food, especially in the end of that season also did not help) and activities that could have been done in the confines of one's house or yard (butchering animals and processing of meat was one of the activities typical for that period). On the other hand, this was also a period when the tree-felling and transport was often scheduled, precisely because of the lower work burden for the local peasants, as no matter how pressing were other matters, production of food was given priority. Likewise, work in cities was less reliant on the natural clock, but still, it was largely limited to the sunrise-sunset diurnal rhythms.

Birrell, J., Peasant Craftsmen in the Medieval Forest in: The Agricultural History Review, vol. 17, no. 2 (1969), pp. 91-107.

le Goff, J., Time, Work and Culture in the Middle Ages, University of Chicago Press, 1982.

Pilkington, J., The Works of James Pilkington, B. D., Lord Bishop of Durham, HardPress Publishing, 2013.

Singman, J.L., Daily Life in Medieval Europe, The Greenwood Press, 1999.