This is a small fraction of all the studies done about the effectiveness of Low Level LED Light Therapy:

Fan L, Yin R, Lan T, Hamblin MR. Photodynamic therapy for rosacea in Chinese patients.Photodiagnosis Photodyn Ther.2018 Dec;24:82-87. doi: 10.1016/j.pdpdt.2018.08.005. Epub 2018 Aug 15

Glynis Ablon, MD, FAAD. Phototherapy with Light Emitting Diodes. J Clin Aesthet Dermatol. 2018 Feb; 11(2): 21–27.J Clin Aesthet Dermatol. 2018 Feb; 11(2): 21–27

Daniel Barolet, MD. Light-Emitting Diodes (LEDs) in Dermatology. 2008; Semin Cutan Med Surg 27:227-238 

Elisabetta Sorbellini, Mariangela Rucco, Fabio Rinaldi.Photodynamic and photobiological effects of light-emitting diode (LED) therapy in dermatological disease: an update.Lasers Med Sci. 2018; 33(7): 1431–1439

Alster TS, Wanitphakdeedecha R. Improvement of postfractional laser erythema with light-emitting diode photomodulation. Dermatol Surg. 2009 May;35(5):813-5. doi: 10.1111/j.1524-4725.2009.01137.x. Epub 2009 Apr 6.

Nancy T. Sacono MDS  Carlos A.S. Costa PhD  Vanderlei S. Bagnato PhD  Fabio C.B. Abreu‐e‐Lima PhD. Light‐emitting diode therapy in chemotherapy‐induced mucositis. Lesers Surg. Med. 40:625–633, 2008.

Harry T. Whelan, Robert L. SmitsJr., Ellen V. Buchman, Noel T. Whelan, Scott G. Turner, David A. Margolis, Vita Cevenini, Helen Stinson, Ron Ignatius, Todd Martin, Joan Cwiklinski, Alan F. Philippi, William R. Graf, Brian Hodgson, Lisa Gould, Mary Kane, Gina Chen, and James Caviness. Effect of NASA Light-Emitting Diode Irradiation on Wound Healing. Journal of Clinical Laser Medicine & SurgeryVol. 19, No. 6. 2004 July.

Shin-ichiro Takezaki, Tokuya Omi, Shigeru Sato, Seiji Kawana. Light-emitting Diode Phototherapy at 630 ± 3 nm Increases Local Levels of Skin-homing T-cells in Human Subjects. Journal of Nippon Medical School. 2006 Volume 73 Issue 2 Pages 75-81

Daniel N. Sauder MD, FAAD, FACP, FRCPC. Light‐emitting diodes: their role in skin rejuvenation. International Journal of Dermatology. Dec 2009

Robert A.Weiss, Roy G.Geronemus, David H.McDaniel. Chapter 12 - LED Low-Level Light Photomodulation for Reversal of Photoaging. Cosmetics Applications of Laser & Light-Based Systems. 2009, 271-280

Robert A. Weiss MD, David H. McDaniel MD, Roy G. Geronemus MD, A. WEISS MARGARET MD, L. BEASLEY KAREN MD, Girish M. Munavalli MD, Supriya G. Bellew MD. Clinical Experience with Light‐Emitting Diode (LED) Photomodulation. Dermatologic Surgery. March 2006

Opel DR, Hagstrom E, Pace AK, Sisto K, Hirano-Ali SA, Desai S, Swan J. Light-emitting Diodes: A Brief Review and Clinical Experience. J Clin Aesthet Dermatol. 2015 Jun;8(6):36-44

Elke M. Vinck,Barbara J. Cagnie, Maria J. Cornelissen, Heidi A. Declercq, Dirk C. Cambier. Increased fibroblast proliferation induced by light emitting diode and low power laser irradiation. Lasers in Medical Science. May 2003, Volume 18, Issue 2, pp 95–99