Two genres that answer different questions
Chinese writing about dreams is usually presented as one tradition with one answer. It is closer to two traditions that answer different questions and do not consult each other. One of them, a single chapter in the 靈樞, asks whether the body is in excess or deficiency and answers with a needle technique. The other, the popular dictionary genre, asks what a dream foretells and answers with a verdict. The pages in this topic are built around that split rather than around a merged summary of it, because the split is where the checkable content is.
The chapter with a mechanism
淫邪發夢, chapter 43 of the 靈樞, opens by explaining why dreams occur — 正邪從外襲內，而未有定舍，反淫於藏，不得定處，與營衛俱行，而與魂魄飛揚，使人臥不得安而喜夢 — and then lists twenty-seven dream scenes. Twelve describe an excess, marked by 盛; fifteen describe a deficiency, marked by 客. Each row names a scene, states the pattern, and ends in an instruction: 寫之, drain, or 補之, supplement. Several images appear twice with opposite readings. Water is 陰氣盛，則夢涉大水而恐懼 in one row and 客於腎，則夢臨淵，沒居水中 in the other; the lung appears as 肺氣盛，則夢恐懼，哭泣，飛揚 and again as 客於肺，則夢飛揚，見金鐵之奇物. A dictionary cannot reproduce that, because a dictionary has one row per symbol. The common dream symbols page works through the disagreement in detail.
The catalogue without one
The dictionary genre, of which 周公解夢 is the best-known survivor, has twenty-seven categories and no theory. Each entry is a four-character dream scene followed by a verdict, and the entries do not agree with each other. Teeth falling out is entered twice with opposite verdicts: 齒自落者父母凶 and 齒落更生子孫興. The water entries split across fortune, imprisonment and bereavement. The snake entries read the colour of the snake — 蛇赤黑口舌青吉 — which means the entry is not about snakes. The page is signed to 周公旦, but its opening verse cites a Southern Dynasties figure and two Tang stories, so the attribution is a convention of the genre rather than a date. Read closely, the dictionary is a catalogue of scenes, and the scenes are where its content sits.
The ritual list, and its second catalogue
The only classical taxonomy of dream types is a job description. 占夢：掌其歲時，觀天地之會，辨陰陽之氣。以日月星辰占六夢之吉凶 — the officer of dreams keeps the calendar and judges six dream types by the sun, the moon and the stars. The paragraph ends in a state ritual: the king’s dreams are collected in the last month of winter, the auspicious ones presented to him, the bad ones sent to the four directions. The six names arrive undefined, and the glosses that fill them in are commentary written centuries later. A few pages earlier the same manual gives the chief diviner a second dream list — 致夢, 觭夢, 咸陟, with ten headings and ninety branches — that shares no item with the first. The dream categories page sets the two catalogues side by side and asks what a classification is for.
Where the medical reading goes
Modern sleep medicine runs the relationship the other way round: it treats the repeated nightmare as a symptom rather than as a message. MedlinePlus lists what repeated nightmares may be a sign of — a breathing disorder in sleep, post-traumatic stress disorder, more severe anxiety disorders or depression, other sleep disorders — and what triggers a run of them, from a newly prescribed medicine to a late meal. It also gives a frequency threshold: contact a provider if nightmares occur more than once a week. One dream behaviour does function as a clinical marker. Acting a dream out is REM sleep behaviour disorder, which MedlinePlus describes as vivid dreaming, talking, violent movements or falling out of bed, and places among the early symptoms of Lewy body dementia, where it can appear years before anything else. That is the strongest link in the whole area between a dream phenomenon and a physical disease, and it is about behaviour during sleep rather than about what a dream meant. The health page sets the classical reading beside it and corrects the organ table that circulates online.
Which page to read first
Start with the page whose question you actually have. If a specific dream is bothering you and you want to know what tradition makes of it, the common dream symbols page is the one that shows the two genres disagreeing about the same image. If you have been handed a list of dream types and want to know where it came from, dream categories. If a dream keeps returning or you act it out, health and dreams, because that is the only page in the topic whose next step is a clinician rather than a notebook. And if you are going to keep a record, recording and interpreting dreams, which is about what a journal measures rather than what it decodes. Read in that order the topic makes one argument: a dream is a reason to look at the evening, the room and the breathing, and a poor reason to conclude anything on its own.