We have known for a long time that the day residue effect exists; this is the term that Freud gave to the memories from the previous day that crop up in that night’s dreams.
Many decades later, oneirologists discovered another time-dependent way in which memories turn up in dreams. As well as appearing in dreams of the same night, they then take several days off in which they don’t appear in any dreams, and then appear again about a week later. This is called ‘dream lag’ (Nielsen & Powell, 1989). This is a ‘circaspetan’ pattern, i.e. every seven days (roughly). When we dream of it the second time around, it may have become more abstract (Blagrove et al., 2011).
Dream lag may occur because sometimes we may need to dream of something repeatedly before we have successfully integrated the experience in our brains (Nielsen et al., 2004). The reason it waits for a week before appearing again is still up for debate, but it might be because this is how long it takes for memories to solidify. When we create a new memory, that memory is initially ‘stored’ in the hippocampus; but in order for it to become a long-term memory, it needs to migrate to the neocortex for permanent storage. This memory migration process takes about a week, so it might be that dream lag dreams reflect this process; the first time the memory appears it’s a hippocampus-based memory, and the second time it appears it’s successfully migrated to the neocortex.
Blagrove, M., Henley-Einion, J., Barnett, A., Edwards, D., & Seage, H. C. (2011). A replication of the 5–7 day dream-lag effect & affect with comparison of dreams to future events as control for baseline matching. Consciousness & Cognition, 20 (2), Jun, pp. 384-391.
Nielsen, T. A., Kuiken, D., Alain, G., Stenstrom, P., & Powell, R. A. (2004). Immediate and delayed incorporations of events into dreams: further replication and implications for dream function. Journal of Sleep Research. 13 (4), Dec, pp. 327-336.
Nielsen, T. A. & Powell, R. A. (1989). The dream-lag effect: a 6-day temporal delay in dream content incorporation. Psychiatric Journal of the University of Ottawa, 14, pp. 561-565.