If you are a newly-qualified teacher starting your first year in school, prepare for bad news: your happiness levels are going to plummet until Christmas.
The change of mood is revealed in a graph which plots the school year against a "positiveness" scale ranging from zero to 120 points.
The graph features in Helping Teachers Develop, a new book by Sara Bubb, an academic at London university's institute of education who is The TES agony aunt for new teachers.
She estimates that NQTs start with a positiveness score of 80, which sinks from September to November, then plunges to zero in the run-up to Christmas.
"They start on a high in September, but then reality strikes and they live from day to day, needing quick fixes and tips for survival," she writes.
"Behaviour management is of particular concern, but they're too stressed and busy to reflect. Colds and sore throats seem permanent."
Luckily, happiness levels should rise from January onwards, reaching the 60 mark around Easter and soaring past 100 by the end of the summer term.
Experienced teachers also face similar swings.
"In January, pupils return calmer and ready to work," Ms Bubb said.
"Teachers can identify difficulties and think of solutions. Then they feel they are mastering teaching and begin to enjoy it."
The graph was inspired by one produced for American teachers by Ellen Moir, of the University of California, Santa Cruz, which Ms Bubb has adjusted for the UK.
"Teachers always seem relieved when I show them the graph, because they realise they are not the only ones who have felt like that," she said.
Helping Teachers Develop is published on September 9 by Paul Chapman Publishing at pound;15.99