There is the simple observation that one has no conscious experience during dreamless sleep. (A panpsychist could respond that maybe one merely lacks memory of one's sleeping experience, but that would be epicyclic).

I like the idea! If you have it, the question is when does it start? Let's look at it for different aspects of consciousness:

• Sentience (Bentham, Singer): Behavioral responses to pleasure or pain stimuli and physiological measures. This is observable across animal species, from mammals to some invertebrates and it should be known when responses to such stimuli start in the embryo.

• Wakefulness: Measureable in virtually all animals with a central nervous system by physiological indicators such as EEG, REM, and muscle tone. The fetus is known to have a sleep wake rhythm, but I don't know when it starts.

• Dennet's Intentionality: Treating living beings as if they have beliefs and desires makes good predictions for many animal species, esp. social, like primates, cetaceans, and birds. Infants show goal directed behaviors right after birth. I remember ultrasound photos that show babies suckling their thumb. I think we can identify when the nervous system is first capable of goal direction.

• Dehaene's Phenomenal Consciousness: A perception or thought is conscious if you can report on it. As this requires language or measuring neural patterns that are similar to humans during comparable reports, I think this starts when communicable representstions of perceptions first form, for toddlers around age one at the earliest with baby sign language.

• Gallup's Self-Consciousness: Recognition of oneself e.g. in a mirror. Requires sufficient sensual resolution and intelligence for a self-model. Dito.

• Rosenthal's Meta-Consciousness: This is investigated through introspective reports on self-awareness of cognitive processes or self-reflective behaviors. Requires more abstraction. Maybe at age five?

Note that "continuous" does not need to be "linear" or similar to that. Maybe the qualia decrease exponentially with the complexity of the structure that experiences them, so maybe each particle has a technically non-zero quality, but still all particles in the universe together have less of an experience than a single human. Numbers can be technically non-zero, and yet zero-ish for most practical purposes.

Some advanced meditators report that they do perceive experience as being basically discrete, flickering in and out of existence at a very high frequency(which is why it might appear continuous without sufficient attention). See e.g. https://www.mctb.org/mctb2/table-of-contents/part-i-the-fundamentals/5-the-three-characteristics/

It really helps if we just taboo the word "consciousness" because people have too many implicit associations wrapped up in what they want that word to mean.

On a day to day level, we want "conscious" be to a stand-in for something like "things that have subjective experiences like mine". This is unfortunately not very useful, as the world is not carved up into thing that are like this and not, other than for other humans.

On the other, if we try to get technical about what we mean for things to be conscious, we either end up at panpsychism by deflating the notion of consciousness (I'm personally supportive of this and think in many cases we should use "consciousness" to refer to negative-feedback control systems because these are the smallest unit of organization that has subjective information), or we end up with convoluted definitions of consciousness to add on enough qualifiers to avoid deflation.

"Consciousness" is a word people are really confused about and have lots of different competing intuitions about what it should mean and I really wish we'd just stop saying it and talk about what we mean directly instead.

Panpsychism is correct about genuineness and subjectivity of experiences, but you can quantize your caring about other differences between experiences of human and zygote however you want.