Speaking as one who lives a mix of long (15-16 hours) and short (25 mins) sectors, I am of the opinion that a "high sector" day is much more tiring than a single long sector.
There's a reasonable "adrenalin factor" for me in operating 4 X 25 minute sectors (Singapore - Kuala Lumpur) in a day, which keeps up the level of alertness. Oddly, on the other side of the coin, a Singapore Amsterdam / Manchester is not so fatiguing because of relief crew, and the mental approach of settling in for the long haul. What I do find fatiguing is a medium length day return (i.e. 2 sectors) of 3.5 to 4 hours each way, such as Singapore - Manila / Hong Kong, particularly if it's 'back of the clock'.
That's just a subjective report, the way that it affects me. But then, it's all legal isn't it?, so I must be just imagining it!
Regards,
Old Smokey