A recent study in The International Journal of Cancer found that night shift work is associated with chronic lymphocytic leukemia (CLL).1 Those who worked night shifts for more than 20 years had a significantly greater risk of CLL, in contrast with day workers.

This association was seen only among those who worked rotating shifts that required alternating schedules of nights and days, but not among those with permanent night schedules.

There are few known risk factors for CLL, and much of the data is inconsistent or unconfirmed. The largest study of independent risk factors found and confirmed a history of farming and of work as a hairdresser to be among the modifiable risk factors positively associated with CLL, suggesting as of yet unidentified environmental triggers.2

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Night shift work was not included in this large pooled analysis, but, as lead author Dr Costas comments, “if confirmed in other studies, this association [between CLL and rotating night work] could be relevant, as it offers a way of reducing the burden of cancer…it could result in preventive actions addressed to reduce its incidence.”

These 3 factors—farming, hairdressing, and night shift work—are traditionally related to low socioeconomic status, and if an association between these and CLL were confirmed, an intriguing and unique relationship between cancer and the social determinants of health would take shape.

The other hematologic malignancies known to have such a relationship, such as HIV-related lymphomas, are of a very different nature, involving classic HIV risk factors and virally mediated pathophysiology. This would invigorate the fields of research involving the pathogenesis of aberrant lymphoid tissue in the socioeconomic poverty setting.

The search for modifiable risk factors begs the question about methods to best mitigate the risk of developing CLL. For it to be more than of just academic interest, oncologists should search for ways to reduce risk for the greater population.

RELATED: CLL-IPI Validated and Compared With MDACC Index in Newly Diagnosed Patients

As Dr Costas notes, “Complete elimination of night shifts is virtually impossible due to the society needs (e.g., health care 24 hours/day). It is therefore important to clarify the role of night shift on cancer, and implement shift systems that minimize circadian disruption.”

If these findings are confirmed, 1 logical policy initiative would be to limit rotating night shifts to less than 20 years in total. Further research into the environmental triggers relating to farming and hairdressing would need to be done to institute similar changes.

Although much is left to be elucidated, this study is a step towards better understanding the causal factors and etiology of CLL.


  1. Costas L, Benavente Y, Olmedo-Requena R, et al. Night shift work and chronic lymphocytic leukemia in the MCC-Spain case-control study. Int J Cancer. 2016 Jul 14. doi: 10.1002/ijc.30272 [Epub ahead of print]
  2. Slager SL, Benavente Y, Blair A, et al. Medical history, lifestyle, family history, and occupational risk factors for chronic lymphocytic leukemia/small lymphocytic lymphoma: the InterLymph Non-Hodgkin Lymphoma Subtypes Project. J Natl Cancer Inst Monogr. 2014(48):41-51. doi: 10.1093/jncimonographs/lgu001