This combination is designed to help alleviate anxiety and placate nervous individuals. Normal doses should not lead to sleepiness, nor the inability to drive etc. but can aid with sleep if taken at night.
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L-tryptophan is an amino acid that helps your body make proteins and control some chemicals in the brain. The body changes L-Tryptophan into serotonin which helps control mood and sleep. It has also been shown to help control anxiety and stress levels by reducing the hormone cortisol.  Additionally, clinical studies have shown that L-Tryptophan actually has a lasting effect on the brain in reducing stress and cortisol levels.
GABA (Gamma-aminobutyric acid) is a neurotransmitter that blocks impulses between nerve cells and the brain. It works by inhibiting the neural cells in the brain and central nervous system which in turn reduces stress, helping to create a calm mood, and helping maintain mental balance. Studies have also shown that GABA not only reduces anxiety, but also helps to induce relaxation.
Magnesium maintaining healthy levels of magnesium helps the nervous system resist feeling stressed. Magnesium increases GABA levels in the body. Magnesium also helps to restrict the release of stress hormone and actually helps to filter them and prevent them from entering the brain. Magnesium also helps the brain’s ability to heal itself and make new neural connections (Brain plasticity/neuroplasticity) which can help teach your brain to become less anxious.  
B6 helps the body produce serotonin (mood regulator).
 The Effects of Dietary Tryptophan on Affective Disorders Glenda Lindseth, Brian Helland, and Julie CaspersArch Psychiatr Nurs. Author manuscript; available in PMC 2016 Apr 1.Published in final edited form as: Arch Psychiatr Nurs. 2015 Apr; 29(2): 102–107. Published online 2014 Dec 9. doi: 10.1016/j.apnu.2014.11.008
 L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications Dawn M Richard, Michael A Dawes, Charles W Mathias, Ashley Acheson, Nathalie Hill-Kapturczak, and Donald M Dougherty, Int J Tryptophan Res. 2009; 2: 45–60. Published online 2009 Mar 23.
 Dietary l-tryptophan leaves a lasting impression on the brain and the stress response Erik Höglund (a1) (a2), Øyvind Øverli (a3), Madelene Å. Andersson (a4), Patricia Silva (a3) (a4) https://doi.org/10.1017/S0007114517001428Published online: 19 June 2017
 Effects of Elevation of Brain Magnesium on Fear Conditioning, Fear Extinction, and Synaptic Plasticity in the Infralimbic Prefrontal Cortex and Lateral Amygdala Nashat Abumaria, Bin Yin, Ling Zhang, Xiang-Yao Li, Tao Chen, Giannina Descalzi, Liangfang Zhao, Matae Ahn, Lin Luo, Chen Ran, Min Zhuo and Guosong Liu Journal of Neuroscience 19 October 2011, 31 (42) 14871-14881; DOI: https://doi.org/10.1523/JNEUROSCI.3782-11.2011
 Magnesium in Prevention and Therapy Uwe Gröber,1,* Joachim Schmidt,1 and Klaus Kisters1,2 1Academy of Micronutrient Medicine, Essen 45130, Germany 2Department of Internal Medicine I, St. Anna-Hospital, Herne 44649, Germany